the role of sulphur in preventing bed agglomeration during ... · increase sharply with...

The role of sulphur in preventing bed agglomeration during

combustion of biomass

Sigrid De Geyter

Degree work presented to obtain a degree in Master of Science in Energy Engineering

Umeå, January 2006

Industrial supervisor: Christer Andersson, Vattenfall Utveckling AB, Älvkarleby University supervisor: Marcus Öhman, Energy Technology and Thermal Process Energy, Umeå University

Abstract In Sweden, incentives are given by the government to phase out the use of fossil fuels such as coal from power plants. Often coal and biomass are co-combusted due to the positive effects of coal on deposit formation and agglomeration characteristics. In spite of many advantages, the main technical challenge for the fluidized bed combustion process is still to minimize the risk on bed agglomeration. Co-combustion with coal has a positive effect on the agglomeration characteristics but whether this is due to its higher sulphur content or to other specific ash properties has not been investigated previously. Apart from coal, other sulphur-rich fuels (such as peat) have also shown to increase the agglomeration temperature during fluidized bed co-combustion with biomass. The aim of this degree work is to investigate the role o f sulphur in preventing bed agglomeration during biomass combustion. The first part of the degree work contains an extensive literature survey on the factors affecting agglomeration characteristics during fluidized bed processes with special attention to what is known about the effect of coal co- combustion and sulphur. Possible methods of agglomeration prevention are reviewed. In a second part, a case study is presented, in order to asses the effect of coal co-combustion on a full- scale example. The experimental part describes the performed laboratory-scale combustion experiments allowing evaluation of the agglomeration characteristics with different forms of sulphur addition during the combustion process as well as of a suggested post- treatment of real-scale bed material with SO2. The level of sulphur-addition was defined prior to the experiments using thermo -chemical model calculations. The bed materials produced in the laboratory experiments are further investigated using SEM/EDS analysis allowing evaluation of the differences in elemental compositions between the different fuels and forms of sulphur addition. A clear difference is found between alkali-rich biomass fuels, used as model for agricultural residue on the one hand and woody biomass fuels rich in calcium but with relatively low alkali-levels in the fuel ash on the other hand. Sulphur treatment is shown to have a clear positive effect during combustion of alkali-rich fuels and almost no effect at the tested levels during combustion of woody biomass. The results of combustion and agglomeration experiments, flue gas measurements and SEM/EDS analysis of the bed material are combined to suggest possible mechanisms for the interaction of alkali and sulphur during combustion.

I would like to thank Vattenfall Utveckling for giving me the opportunity to study agglomeration more in detail and for supporting my experiments, Christer Andersson for his supervision and the interesting discussions we had throughout the project. Further thanks to Marcus Öhman for his confidence and supportive supervision, and to my colleagues at ETPC who helped with the practical work in the laboratory. I also thank Elisabeth Brus for letting me “inherit” her extensive literature database on agglomeration phenomena.

Table of contents

1 INTRODUCTION..............................................................................................................................................................................1

1.1 BACKGROUND............................................................................................................................................................................... 1 1.2 OBJECTIVES OF THE STUDY......................................................................................................................................................... 1

2 THEORY AND LITERATURE REVIEW .................................................................................................................................2

2.1 FLUIDIZATION, DEFLUIDIZATION AND AGGLOMERATION....................................................................................................... 2 2.2 LITERATURE REVIEW................................................................................................................................................................... 4

2.2.1 Mechanisms involved in defluidization and agglomeration during biomass combustion in a fluid bed.............4 2.2.2 The effect of process parameters on defluidization and agglomeration ...................................................................6 2.2.3 Effect of fuel composition..................................................................................................................................................7

2.3 MEASURES FOR PREVENTION.................................................................................................................................................... 10 2.3.1 Measures based on ash reaction chemistry..................................................................................................................10 2.3.2 Process control-based measures: early warning systems..........................................................................................17

3 A PRACTICAL EXAMPLE: THE IDBÄCKEN PLANT....................................................................................................19

3.1 HISTORY....................................................................................................................................................................................... 19 3.2 TECHNICAL DATA....................................................................................................................................................................... 19

3.2.1 P1 and P2...........................................................................................................................................................................19 3.2.2 P3.........................................................................................................................................................................................19

3.3 ASH RELATED PROBLEMS AT THE IDBÄCKEN PLANT............................................................................................................. 21

4 MATERIALS AND METHOD ....................................................................................................................................................22

4.1 HYPOTHESES AT THE BASIS OF THE EXPERIMENT S................................................................................................................ 22 4.2 THERMO-CHEMICAL EQUILIBRIUM CALCULATIONS.............................................................................................................. 22 4.3 BENCH-SCALE COMBUSTION EXPERIMENTS........................................................................................................................... 24

4.3.1 The CFBA-method............................................................................................................................................................24 4.3.2 Fuels....................................................................................................................................................................................25

4.4 ADDITIVES AND OVERVIEW OF EXPERIMENTS........................................................................................................................ 25 4.5 FLUE GAS ANALYSIS................................................................................................................................................................... 26 4.6 SEM/EDS ANALYSIS OF BED MATERIAL ................................................................................................................................ 26

5 RESULTS ...........................................................................................................................................................................................28

5.1 THERMO-CHEMICAL EQUILIBRIUM CALCULATIONS.............................................................................................................. 28 5.2 AGGLOMERATION TEMPERATURES.......................................................................................................................................... 30 5.3 FLUE GAS ANALYSES.................................................................................................................................................................. 31 5.4 SEM/EDS -ANALYSES................................................................................................................................................................ 32

6 DISCUSSION ....................................................................................................................................................................................35

6.1 IDBÄCKEN AND COAL ADDITION............................................................................................................................................... 35 6.2 SULPHUR ADDITION TO MODEL FUELS..................................................................................................................................... 35

7 CONCLUSIONS ...............................................................................................................................................................................39

7.1 IDBÄCKEN FUEL .......................................................................................................................................................................... 39 7.2 MODEL FUELS............................................................................................................................................................................. 39

REFERENCES ..........................................................................................................................................................................................40

THE ROLE OF S IN PREVENTING BED AGGLOMERATION DURING COMBUSTION OF BIOMASS

1

1 Introduction

1.1 Background Traditional “clean” wood fuels have been extensively used in fluidized bed combustion (FBC) with limited problems. The increasing share of recently introduced fuels such as wood-based residues, logging debris, agricultural crops and different municipal and industrial residues have however lead to an increasing number of often ash-related operational difficulties. Ash-related problems can be subdivided into different categories according to their location in the installation. Most important problems are classified as fouling, slagging, high temperature corrosion and bed agglomeration. This work is focusing on the latter. In order to prevent bed agglomeration and sintering coal can be added to a “difficult” biomass fuel mix, as this has been demonstrated to be an effective means of reducing the agglomeration risk (Nordin, Öhman et al., 1995). There are however several reasons of both environmental and in some countries even economical character why it is interesting for Combined Heat and Power (CHP) plants to decrease the share of coal in their fuel mix, and replace it by alternative measures of agglomeration prevention (Andersson, Andersson et al.; Brus, 2000; Vattenfall, 2004): 1. coal is a fossil fuel, its combustion is contributing to the greenhouse effect with a net CO2-production; 2. non-neutral CO2-emissions make coal combustion subject to CO2- taxes; 3. some governments (e.g. Sweden) apply green electricity certificates penalizing the use of fossil fuel to produce electricity The mechanisms by which coal addition positively influences the agglomeration processes are not yet fully understood. Coal is significantly richer in sulphur content than woody biomass. Whether it is the sulphur content itself, or other coal-specific properties, such as their content in minerals (mainly clay and carbonates) that cause this positive effect has not been studied in detail previously. Sulphur has other demonstrated positive as well as negative effects during combustion processes. In order to minimise SO2 concentrations in the flue gases during the combustion of high-S coals, the use of additives such as limestone and dolomite is widely used. Paradoxically, the ability of sulphur to react with alkali-elements in the fuel ash has also been successfully implemented to reduce fouling and high temperature corrosion problems. In this case, sulphur addition to the fuel or to the furnace during combustion of low-S, high-alkali fuel prevents the formation of troublesome alkali-chlorides (Berg, Andersson et al., 2003). Furthermore, sulphate has been found to have a positive effect on emissions by lowering the CO- and TOC-content of the flue gases. Also NOx emissions could be reduced by a reduced air excess as well as by increasing the efficiency of the SNCR upon addition of ammonium sulphate (Lindau and Skog, 2003; Kassman, Andersson et al., 2005). The role of sulphur in the mechanisms involved in bed agglomeration are however less well- known. A specific study about the effect of sulphur on agglomeration and defluidization mechanisms has to the authors knowledge not yet been reported. Hence it is not known whether sulphur addition might be an alternative to coal co-combustion in order to prevent agglomeration. It is therefore of interest to get an understanding of whether and in which form sulphur addition could influence the risk for bed agglomeration.

1.2 Objectives of the study 1. to give an overview of the state-of-the-art of the knowledge on bed agglomeration mechanisms and preventive measures 2. to evaluate the potential risk for agglomeration when reducing coal co-combustion in the fuel mix at the Idbäcken plant in

Nyköping 3. to evaluate the role of sulphur in preventing bed agglomeration during the combustion of biomass, by adding sulphur in different

forms to the combustion zone


2

2 Theory and literature review

2.1 Fluidization, defluidization and agglomeration In a fluidized bed reactor, a mixture between gas and solid particles can cause different types of flow profiles. At low gas velocities, particles are not moving, while the gas flows in between. This type of flow profile is called a fixed bed. At increasing gas velocities, the pressure drop over the bed will increase and as soon as it corresponds to the weight of the sand in the bed, the bed is starting to fluidize. At this point, the minimal fluidization velocity is defined (Umf). If gas velocity is further increased, the particles in the reactor will be in free suspension, and the reactor contains a fluidized bed (Simonsson, 1988). Figure 1 and Figure 2 give an overview of the effect of increasing gas velocity on the flow profile and pressure drop over the bed.

Figure 1: Illustration of a) fixed bed, b) maximal expansion of the bed, c) fluidized bed (Simonsson, 1988)

Figure 2: The pressure drop versus gas velocity at the change from fixed to fluidized bed (Simonsson, 1988)

In region between B and C in Figure 2, the bed is expanding until fluidization in point C. Further increase of the gas velocity is no longer affecting the pressure drop beyond this point. Because of high combustion efficiency, low emission levels and good fuel flexibility, combustion in fluidized bed reactors has proven to be an attractive process for utility scale biomass power plants. A relation between pressure drop over the bed area and minimum fluidization velocity is given by Ergun’s equation (Eq. 1)

( )pp

mfp

pmf

mf

pp

mf

mf

mf

du

d

uLp

**

**

1*75,1

*

**

1*150

2

3223

2

ΦΦ

−+

Φ

−=∆ ρ

ε

εµ

ε

ε Eq. 1

Where:


3

L µ umf εmf dp ρp Φp

: Bed height : Viscosity of the gas : Minimal fluidization velocity : Bed porosity at minimum fluidization velocity : Diameter of the particles : Density of the particles : Form factor for particles (1 for spherical, 0<Φp<1 for non-spherical particles

At high temperature, it was shown that the minimum fluidization velocity will not follow the theoretical value calculated by Eq. 1, but increase sharply with temperature. The starting point for this increase in Umf was defined as the “initial sintering temperature” Ts. Thus, the minimum fluidization velocity is also a function of temperature, and it demonstrates two regimes delimited by the initial sintering temperature as shown in Figure 3.

Figure 3: Fluidization diagram at high temperature with and without sintering effects (Lin, Dam-Johansen et al., 2003)

Physically, the cause of this phenomenon has been identified as being the appearance of a liquid phase into the suspension of solid particles in gas. This liquid phase changes the inter-particle forces causing adhesion between particles by neck formation at the point of contact. When this bond becomes strong enough to withstand the disruptive forces in the fluidized bed, the defluidization point is reached (Seville, Silomon-Pflug et al., 1998; Lin, Dam-Johansen et al., 2003; Golriz, Eriksson et al., 2005). As bed materials used during fluid bed combustion on their own often are rather stable solid phases in the temperature domains of FBC operation, the cause for liquid phase formation should be sought elsewhere. Coal and biomass contain certain amounts of ash that could remain as individual ash particles in the bed or react with the bed material to form low melting compounds. Fuel-ash composition will therefore play an important role for the agglomeration process, as many alkali and alkaline earth metals can participate in the formation of low-melting compounds and tend to become sticky, causing an adhesive force between bed particles.


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2.2 Literature review The mechanisms of liquid phase formation, transport to particle surfaces and adhesion phenomena have been intensively studied during the last decades and many attempts are made to find methods for prevention of defluidization and agglomeration problems in full-scale installations. The processes involve systems which are multi -phase and are characterised by complex chemical reactions. Although the primary aim of this work is to focus on the role of sulphur in defluidization mechanisms, part of the task within the frame of this thesis is also to give a rough overview of the extensive literature and state of the art on mechanisms and measures for prevention of agglomeration problems in industrial plants. Throughout this work, special attention has been given to sulphur and elements that are known to react with sulphur during combustion processes. 2.2.1 Mechanisms involved in defluidization and agglomeration during biomass combustion

in a fluid bed In 1988, the first study on bed agglomeration resulting from biomass fuels was published. Since then, many studies have been reported with focus on description of the complex mechanisms of fluidized bed agglomeration. For an extensive literature overview, the reader is referred to (Brus, 2004). Some comprehension of the complex processes is however imperative for the further discussion. In an attempt to summarise the findings from literature and further simplify them based on (Brus, 2004), following important steps (illustrated in Figure 4) can be discerned to be involved in the defluidization process:

• Formation of a liquid phase consisting of low melting alkali silicates and/or salts initiates agglomeration. Temperature is the most important operating variable, whereas mineral composition and mainly alkali content is the most important fuel characteristic in this context;

• The most important adhesion mechanisms are mainly physical processes: surface forces such as van der Waals or electrostatic forces, initial adhesion is subsequently strengthened by sintering;

• Adhesion of bed particles is found to follow two different pathways: either direct or indirect adhesion via initial layer formation. In total, 8 different pathways are suggested.

• Direct adhesion: 1. By solid particles: only occasionally suggested, would have to include a fast solid state reaction or sintering rather

unlikely to occur with the typical contact times between ash and bed particles in fluidized beds 2. By liquid particles: melted particles adhere to bed particles via collision 3. By sticky ash on burning fuel particles: sticky ash particles adhere to bed particles via collision 4. a) Direct chemical reaction sintering, involving simultaneous solid-solid and solid-gas reactions

• Initial layer formation with subsequent adhesion is however dominating (together with 4.a) the mechanistic agglomeration literature:

4. b) Coating initiated chemical reaction sintering: solid-gas reactions followed by adhesion. Both chemical reaction sintering mechanisms (4 a & b) are likely to occur only during conditions of low fluidization or particle movement such as cyclone return legs

5. By melted ash particles, forming a layer around the bed particle, increasing risk for adhesion 6. By melted ash particles, forming an attack layer by reaction with the bed particle 7. Gaseous alkali attack on the bed particles: gas-solid reaction forming an attack layer 8. Particle/aerosol deposition on the bed particles followed by reaction and attack layer formation

From two main routes for agglomeration, also referred to as “melt-induced” and “coating induced”, the latter has been suggested to predominate in agglomeration/defluidization during FBC of biomass fuels: a coating consisting of fuel components is formed on the bed particle and if it contains low melting compounds, this is followed by adhesion and agglomeration. Several superimposed layers often constitute the total bed particle coating, the innermost often similar in composition to that of the adhesive material in agglomerates, with high contents of K and Ca. Layer formation often starts with physical adhesion followed by condensation of gaseous alkali species and reaction with bed material as well as decomposition of alkali sulphates. This is resulting in 3 layers: (i) an outer ash layer formed via sintering of attached particles, growth balanced by bed attrition, (ii) a uniform Ca-silicate layer below the ash layer formed by diffusion of Ca into the quartz, controls the adhesive forces for the temperature-controlled agglomeration process (iii) another uniform silicate layer, rich in K but also containing some Na and Ca. Although many different pathways have been identified, understanding of all the mechanisms involved is still far from complete, and there are significant inconsistencies and uncertainties (Brus, 2004). With increasing variation in bed materials and fuel types being


5

By solid particles

By melted particles

By sticky ash on burning fuel particles

Direct adhesion

1.

2.

3.

4a.

Initial layer formationSubsequent adhesion

Coating formationby melted ashparticles

- “ - followed by attack layer formation

Gaseous alkaliattack/heterog.reaction

Particle/Aerosoldeposition, followed by coating or attack layer formation

5.

6.

7.

8.

Direct chemical reaction sintering

4b. Coating initiated chemical reactionsintering

By solid particles

By melted particles

By sticky ash on burning fuel particles

Direct adhesion

1.

2.

3.

4a.

Initial layer formationSubsequent adhesion

Coating formationby melted ashparticles

- “ - followed by attack layer formation

Gaseous alkaliattack/heterog.reaction

Particle/Aerosoldeposition, followed by coating or attack layer formation

5.

6.

7.

8.

Direct chemical reaction sintering

4b. Coating initiated chemical reactionsintering

introduced on the market, it is therefore still difficult to predict agglomeration risk based on the state-of- the art mechanistic knowledge. Mechanisms 4a ? 8 in Figure 4 were found to dominate agglomeration literature. More specifically for biomass fuels, (Brus, Öhman et al., 2005) found three dominating mechanisms involved in agglomeration: (a) coating induced agglomeration with attack and diffusion of Ca, forming low melting silicates also including minor amounts of K, with subsequent viscous-flow sintering and agglomeration (typical for woody fuels) ; (b) direct attack by K in gas or aerosol phase, forming low melting silicates inducing viscous-flow sintering and agglomeration (typical for alkali-rich fuels) and (c) direct adhesion by partly melted alkali-silicate droplets (fuels with high alkali and reactive silica content). These mechanisms (a) and (b) correspond mainly with schemes 7 and 8, whereas mechanism (c) corresponds with schemes 5 and 6 in Figure 4. Model fuels typical for mechanisms (a) and (b) will be used for evaluation of the effect of sulphur in the experimental part of this work.

Figure 4: Overview of different agglomeration mechanisms (Brus, 2004)


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2.2.2 The effect of process parameters on defluidization and agglomeration 2.2.2.1 Temperature The agglomeration process is a strongly temperature-related process. Unintentional temperature increases caused by overloading, process disturbances or improper fluidization conditions in the bed area might therefore result in defluidization and agglomeration. 2.2.2.2 Gas velocity, bed particle size and age The effect of bed particle size and superficial gas velocity on defluidization during fluidized bed combustion of forestry residue was studied by (Eriksson, Öhman et al., 2004). An overview of the results is obtained from Figure 5.

800

850

900

950

1000

1050

1100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

U - Umf (m/s)

Def

luid

isat

ion

tem

per

atu

re (°

C)

Brista fine

Brista coarse

C4 Energy fine

C4 Energy coarse

Idbäcken

Figure 5: Initial bed defluidization temperature vs U-Umf for different fluidized bed units and particle sizes (Eriksson, Öhman et al., 2004)

Influence of U-Umf on defluidization temperatures is strong at lower fluidization velocities, but seems to become insignificant at higher fluidization velocities U-Umf > 1m/s. In BFB installations, Umf varies around 0,4 – 0,5 m/s for the overall sand material. The mean sand particle size varies between 500-800 µm. In CFB installations, the sand particles are much smaller (150 – 350 µm) and Umf is about 0,1 m/s. The effect of particle size is less obvious. In the case of Brista, the initial agglomeration temperature was increased for small particles, whereas for C4 Energy no effect could be recognised. The fine fraction can be expected to have a lesser proportion of old particles with thick coatings. Previous studies from the Brista plant indeed indicated a decrease of initial defluidization temperature with 30ºC resp. 60ºC after one resp. five days of combustion since complete bed change. The elemental analysis of coatings on small or large particles did not differ. Age of the bed particles and/or particle-size can however play an important role in the operation of a fluidized bed in practice. For example has in some cases evidence of segregation been found, with an enrichment of older bed material in slower circulating areas and of fresh sand at the bottom section of the riser, where the old bed material is being exchanged. Defluidization temperatures of samples of both areas differed with more than 50ºC (Eriksson, Öhman et al., 2004). (Lin, Dam-Johansen et al., 2003) reports a shorter defluidization time at increased initial bed particle sizes. Also, the effect of fluidization velocity was found to be of importance, although only two different velocities were evaluated. 2.2.2.3 Effect of stoichiometry Recently, scientific interest has increasingly turned from combustion to focus more on gasification of biomass, as this opens a whole range of new opportunities for production of chemicals, liquid fuels and power production from biomass. Gasification is confronted with similar ash related problems as combustion. A set of different biomasses was tested with respect to their agglomeration behaviour (Öhman, Pommer et al., 2005) under different air stoichiometries, typical for combustion and gasification processes. For all fuels under investigation, except for Lucerne, no major differences could be detected in agglomeration tendencies or bed particle layer characteristics, indicating no major difference in layer formation processes or agglomeration mechanisms between the two operational modes.


7

For the sulphur-rich Lucerne however, agglomeration temperatures during combustion were signifi cantly lower than during gasification. This was explained by the formation of salt-melts rich in K, S and Cl as well as K2SO4 (s) which according to thermochemical model calculations were predicted to form during combustion but not during gasification. Moreover, structural differences were detected in the coating layer formation. Whereas during combustion only 10% of the bed particle layers were covered with a thin layer and separate fuel ash derived particles were detected, the bed particle layers were thick during gasification. Evidence of the insignificance of combustion stoichiometry on defluidization tendency is also found in other studies (Skrifvars, Hupa et al., 1994; Lin, Krusholm et al., 1997; Lin and Dam-Johansen, 1998). 2.2.3 Effect of fuel composition 2.2.3.1 Specific elements in biomass In a dedicated study (Brus, Öhman et al., 2005) compositions of particle coatings and mechanisms responsible for agglomeration during FBC of biomass were further revealed. The material accumulated on the bed particles consisted of two layers with a composition depending on both fuel and bed material characteristics. The innermost layer seems to be rather homogeneous and similar in composition to agglomerates: high concentrati on of K- and Ca-silicates and low contents of a few other elements. This layer is observed to grow inwards, by attack or chemical reaction of Ca, forming mainly Ca-silicates and K- or other alkali (e.g. Na-) silicates wherever high concentrations of these elements occur in the fuel. The outer layers seem more to resemble the fuel ash composition and consist of a sintered matrix containing micron-sized particles. The overall compositional distributions of the melt surrounding the bed particles are mainly limited (>90%) to different phases of the ternary phase diagram K2O-CaO-SiO2 (in appendix) (Morey, Kracek et al., 1930). Most biomass fuels produce coatings around the quarts particles with a composition close to the SiO2-rich corner of this phase diagram. The first melting temperature in this region is 720°C, which is well below the usual operation temperature during FBC (Öhman, 1999; Brus, 2004). However, small increases in the Ca/(Na+K) ratio are able to shift this value to another eutectic, at about 1080°C (Öhman, Nordin et al., 2003). From the above it is concluded that elements such as K, Na, Ca and Si are playing an important role in the agglomeration process. 2.2.3.2 Coal-combustion and agglomeration Studies on the agglomeration mechanisms during coal combustion started in 1982. During fluidized bed coal combustion, the ash content in the coal is often so high that no external bed material is needed. However, calcite and dolomite often added as sorbents in order to capture SO2, could be regarded as the “bed material” in this case. Although agglomeration problems are only limited with most coal types, some agglomeration might occur, mainly when the coal contains a lot of alkali. Apart from the mechanisms identified under 2.2.1 and the effect of process parameters previously discussed under 2.2.2, some specific characteristics of coal fuels have been found to be of importance. Mainly the content of Na and alkali in the coal has a strong effect on the agglomeration tendency. Pyrite in coal was found to form the low-melting eutectic FeS-FeO. The formation of alkali-sulphates was identified as an intermediate step in the agglomeration process of high alkali and high-S-coals. Also fine particles of SiO2 and Al2O3 were found to play a role (Lind, 1999; Brus, 2004). 2.2.3.3 Effect of high sulphur content in coal and experiences from the sulphation process During combustion of fuels with a high sulphur-content, fluidized beds have the ability to capture SO2 in situ if calcitic limestone or dolomite is added. The reaction of these additives with fuel-S to form sulphates is known as the sulphation process. More than 30 years of intensive study of this process in atmospheric FBC boilers has tried to clarify the reaction mechanisms of sulphur capture. For a complete overview the reader is referred to (Anthony and Granatstein, 2001). An important number of publications is found concerning ash related problems during combustion of high-S fuels, such as low rank coals and petroleum coke (up to 5-8 wt% S) (Anthony and Jia, 2000; Anthony, Jia et al., 2003). The addition of external S to biomass fuels as a method for prevention of agglomeration might therefore seem rather contradictory. Hence it is important to understand which mechanisms previously were discovered to be responsible for agglomeration during FBC of S-rich coals. For high-alkali and high-S lignite, the initial coating layers were found to be rich in Na, Mg, Ca and S, suggesting the presence of sulphates of these compounds. Agglomeration seemed to be highly dependent upon the Na-content of the fuel (Goblirsch, Benson et al., 1983). Outer layers consisted additionally of Si, Al and Fe. Agglomerates had a significant presence of Na, Si and S. Eutectics consisting of silicates and sulphates of Na are known to melt at low temperatures, below 800°C (Bhattacharya and Harttig, 2003). The 3-component system Na2SO4-CaSO4-MgSO4 that was detected in the coating on the bed particles has a eutectic already at 670°C (Manzoori, Lindner et al., 1992). For S-rich coal, Ca is usually added as limestone or dolomite in order to prevent SO2 emissions. For limestone, the following reaction scheme is known:

23 COCaOCaCO +⇔ Eq. 2


8

422 21 CaSOOSOCaO ⇔++

Eq. 3

If dolomite is used, the reaction scheme is described by:

223 2COMgOCaO)CO(CaMg ++⇔ Eq. 4

MgOCaSOOSOMgOCaO +⇔+++ 422 21

Eq. 5

Fouling in such installations has been found to be associated with the sulphation process itself. In some cases the deposits in the installation burning high-S fuels consisted almost entirely of CaSO4. The mechanisms suggested are however mainly associated with limestone or dolomite particle morphology rather than with the CaSO4: chemical reaction sintering, molecular cramming by particle expansion, bulk diffusion, bonding between particles due to transient melts, or formation of CaMg2(SO4)3. Attempts to ascertain if pure CaSO4 would agglomerate have been mostly unsuccessfu l. The characteristics of CaSO4 are probably modified in the process of agglomeration but not thought to be causing agglomeration (Anthony, Jia et al., 2003). Therefore, if elementary or sulphate-S is added to a fuel mix, these effects are not likely to occur, as they seem to be associated with specific (physical) characteristics of the limestone or dolomite particles. Some aspects are however limiting the sulphation process, making it rather inefficient. Apart from the physical structure of the calcited limestone or dolomite particles, other factors limiting the sulphation efficiency might be the shifting oxidising and reduction conditions during FB combustion as well as the observed temperature maximum, illustrated in Figure 6. At typical FBC temperatures (800-950ºC), and oxidising conditions, CaSO4 is the favoured final product of sulphation process and thermodynamically stable, although temperature increase impairs this stability (Figure 6). Locally reductive zones are often present in the bed area and are capable of shifting Eq. 6 to the right, again releasing SO2 and CaO.

CaO(s)

CaSO4(s)

CaS(s)

1100 K1300 K

log10(p(O2)) (atm)

log

10(p

(SO

2))

(atm

)

-16 -14 -12 -10 -8 -6 -4 -2 0-6

-5

-4

-3

-2

-1

0

1

Figure 6: Phase diagram for CaSO4 stability and temperature dependence

OH/COSOCaOH/COCaSO 22224 ++⇔+ Eq. 6

On the other hand, a temperature maximum for S-retention has been recognised. Several explanations have been given to this phenomenon. The consensus view seems to be that a temperature maximum around 850ºC for sulphation is due to a competition


9

between sulphation and reduction reactions that are becoming more important at higher temperatures (Lyngfelt and Leckner, 1989a, 1989b; Anthony and Granatstein, 2001).


10

2.3 Measures for prevention (Zintl, 1997) lists the possible measures for prevention as follows:

• Prevent the occurrence of hot spots, due to improper fluidization

• Reduce the combustion temperature

• Modify the fuel chemistry, in order to reduce content of difficult ash-components (mostly alkali), by: • Fuel refining by pretreatment of the fuel, e.g. select critical parts of the fuel, leaching or harvesting during the spring in

stead of during the autumn • Addition of additives such as calcite, dolomite, bauxite, clay minerals

• Co-combustion with other fuel types, changing the overall ash-chemistry and using synergic effects or dilution effects

• Modification of the bed material, such as choice of specific purity or choice of other bed material composition A today widely used method for prevention of bed agglomeration is continuous bed material exchange. However preventing bed agglomeration rather effectively, this is expensive practice. The cost for bed material exchange, transport and disposal of ashes for a variety of Swedish FB installations shows a yearly cost between 0,05 and 3 Million SEK per year per installation. Furthermore, the list of possible preventative measures should be extended with bed diagnostic tools for early warning on bed agglomeration problems such as proposed by (Nordin, Öhman et al., 1996; van Ommen, Schouten et al., 1999b; van Ommen, Schouten et al., 1999a; van Ommen, Schouten et al., 2001a; van Ommen, Schouten et al., 2001b; van Ommen, Schouten et al., 2001c; Kiel, Korbee et al., 2002; Korbee, Eenkhoorn et al., 2002; Öhman, Hokfors et al., 2002; Korbee, van Ommen et al., 2003). Conclusively, the measures for prevention listed above can be rearranged into three categories: 1. Measures based on process design parameters: fluidization control, process temperature 2. Measures based on ash reaction chemistry: additives, change in bed materials or fuel composition 3. Process control-based measures: early warning systems Process parameters are depending a lot on the design of the installation, which is different for each boiler constructor and specific for each installation. Some of the basic aspects are already discussed under 2.2.2 and will not be further discussed here. 2.3.1 Measures based on ash reaction chemistry 2.3.1.1 Additives Many different additives have been tested during FBC combustion in order to change the chemistry of the combustion environment and reduce the risk for agglomeration. In an attempt to summarise the most important literature in this field, Table 1 gives an overview of additive- fuel combinations and their literature references. Conclusions and findings of these reports are shortly discussed per additive-type.


11

Table 1: Overview of references describing the effect on agglomeration of the use of additives during FBC

Additive Fuel Reference Additive compostion / size Clay Low-rank coal

South Australian lignite

(Linjewile and Manzoori, 1997) (Vuthaluru, Linjewile et al., 1999) (Vuthaluru and Zhang, 2001a, 2001c)

Rich in kaolinite (Al2Si2O5(OH)4) and sillimanite (Al2O3)(SiO2)

Kaosil Low-rank coal (Vuthaluru and Zhang, 2001a, 2001c)

Sillimanite-Kaolinite

Bauxite Low-rank coal (Vuthaluru and Zhang, 2001a, 2001c)

Al-ore

Dolomite South-Australian lignite Bark & sawdust South-Australian lignite MBM1 RDF2 MBM&RDF

(Linjewile and Manzoori, 1997) (Vuthaluru, Linjewile et al., 1999) (Daavitsainen, Nuutinen et al., 2001) (Bhattacharya and Harttig, 2003) (Öhman, Nordin et al., 2003) (Öhman, Nordin et al., 2003) (Öhman, Nordin et al., 2003)

CaMg(CO3)2 0.08-0.2 mm

Clay South-Australian lignite

(Linjewile and Manzoori, 1997) (Vuthaluru, Linjewile et al., 1999)

Rich in kaolinite and quartz

Gibbsite South-Australian lignite

(Linjewile and Manzoori, 1997) (Vuthaluru, Linjewile et al., 1999)

Al(OH)3

Kaolin Forest residue Alfalfa Bark Wheat straw MBM RDF MBM&RDF

(Zintl and Ljungdahl, 2000) (Zintl and Ljungdahl, 2000) (Öhman and Nordin, 2000) (Öhman and Nordin, 2000) (Öhman, Nordin et al., 2003) (Öhman, Nordin et al., 2003) (Öhman, Nordin et al., 2003)

Clay essentially composed of kaolinite Al2Si2O5(OH)4

Kyanite Forest residue Alfalfa

(Zintl and Ljungdahl, 2000)

Al2SiO5

Calcite Forest residue Alfalfa


CaCO3

Sulphurdioxide Biomass (Zintl and Ljungdahl, 2000) SO2 Sand South Australian

lignite (Bhattacharya and Harttig, 2003)

SiO2, fine (0.15-0.25mm) and coarse (0.85-1mm) additive

Aluminiumoxide South Australian lignite

(Bhattacharya and Harttig, 2003) 94,7% Al2O3 (0.075-0.15 mm) 98.5% Al2O3 (0.075-0.15 mm)

Fly ash South Australian lignite

(Bhattacharya and Harttig, 2003) 0.06-0.1 mm

Sillimanite- and kaolinite-rich clay addition resulted in the formation of a moderate deposit around the bed material particles. No S was found in the coating, and the usual, low-melting Na-sulphates were replaced by high melting Na-aluminosilicates (Linjewile and Manzoori, 1997). This clay type was found to reduce the formation of ash deposits on bed particles significantly, compared to experiments without additive (Vuthaluru, Linjewile et al., 1999). Dolomite addition produced a thick coating layer on the bed particles. The ash coating contained sulphated dolomite that by diluting the low melting NaSO4 eutectics anyway resulted in an overall lowered tendency to agglomerate (Linjewile and Manzoori, 1997; Vuthaluru, Linjewile et al., 1999). On the other hand, an important drawback for dolomite addition is the important ash build-up on the sand particles (Vuthaluru, Linjewile et al., 1999). The formation of several superimposed coating layers during combustion of bark and saw dust was also recognised by (Daavitsainen, Nuutinen et al., 2001) Clay additive rich in kaolinite and quartz resulted in thick and strongly bonded ash deposits on the bed particles and even here, S was absent from the coating. The coating consisted mainly of an amorphous phase attributed to glassy Na-silicates, as a result of reactions between quartz in the clay and Na in the coal. As the molten phase solidifies at the bed temperature, this additive reduces agglomeration tendencies (Linjewile and Manzoori, 1997; Vuthaluru, Linjewile et al., 1999). Gibbsite however effectively prevents the formation of significant ash coating on bed particles (Linjewile and Manzoori, 1997; Vuthaluru, Linjewile et al., 1999). The existing coating layer was enriched in Al and even S. Na2SO4 was detected and Na did not

1 MBM: Meat and Bone Meal 2 RDF: Refuse Derived Fuel


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seem to have reacted with Al to form amorphous Na aluminate. The mechanism by which Gibbsite is preventing ash deposition on the bed particles was attributed to the fact that heat- treated gibbsite transforms to activated alumina with a high internal surface area. The high porosity of the alumina particles allows molten ash constituents to be absorbed (Linjewile and Manzoori, 1997). Clay, kaosil and bauxite, showed coatings on the sand bed particles enriched in Al, Si, Na and S. Additives were shown to have reacted with Na to form high melting silicates. These additives were found to be effective in controlling agglomeration and defluidization, enriching the ash deposition on the bed particles in Al and Si (Vuthaluru and Zhang, 2001a, 2001c). The addition of 2-5%(w to fuel) kaolin extended combustion time of alfalfa without agglomeration from 15minutes without additive to more than 11 hours with 5% kaolin. The effect of kaolin in reducing agglomeration risk is explained on the basis of its unique structure and chemical composition: during combustion, -OH groups are vaporised, leaving vacant holes in the structure for alkali-ions to form KAlSiO4, KAl2Si3AlO10(OH)2 and NaAlSiO4. The agglomeration risk was also even further reduced for the less agglomeration-sensitive forest residues upon addition of kaolin (Zintl and Ljungdahl, 2000). 10%(w to bed) kaolin added to the bed material was reported to form small aggregates (<1mm) probably originating from the transformation of kaolin into meta-kaolinite (more or less amorphous structures of Al2O3*2SiO2). The outer surfaces of the aggregates contained some Si-, Al- and K-oxide rich particles. Based on the relatively high melting temperature of the particles in the agglomerates, these were not found to participate in the agglomeration process. Introduction of kaolinite resulted in adsorption of K from the biomass fuels onto the aggregates, thus lowering its concentration in the bed material coatings. Theoretically deduced melting temperatures, based on the elemental composition of the bed material coating and the K2O-CaO-SiO2 ternary diagram, were in accordance with experimentally measured agglomeration temperatures, in that the initial agglomeration temperature was increased with about 100°C for wheat straw and 10°C for bark by kaolin introduction. With kyanite as additive, there was no improvement on agglomeration behaviour neither for alfalfa nor for forest residues. The effect of kaolin was therefore confirmed to be attributed to its specific structure, rather than to its chemical composition(Zintl and Ljungdahl, 2000). Calcite was used to test the effect of an increased Ca/K-ratio. For alfalfa, an improvement was observed, whereas for forest residue, agglomeration occurred directly upon introduction of calcite to the furnace, which could mainly be attributed to the higher test temperature (1050°C, in stead of 850°C for alfalfa) used for this fuel. Calcite as an additive is furthermore not favoured because calcination upon introduction to the furnace can be followed by exothermal hydration, which may lead to local high temperature development in the furnace (Zintl and Ljungdahl, 2000). In most reports, the effect of additives on agglomeration risk has been attributed to chemical reactions preventing coating formation, forming higher-melting eutectics and/or diluting low-melting ones. However, for agglomeration-sensitive high-alkali, high-S lignite, it was established that any additive which is of fine size and itself is not a low-melting-point compound can potentially control defluidization in a CFBC. Fine sand, aluminiumoxide and fly ash all effectively prevented agglomeration, whereas defluidization occurred when course sand was used. Fine additives provide large surface areas and capture the fine ash or agglomerate-forming constituents, which are more easily elutriated from the bed, thereby controlling agglomeration and preventing defluidization (Bhattacharya and Harttig, 2003). Moreover, it was shown that the use of additives reduced the levels of Na, Mg, Ca, S and Cl in the coating layers. For full scale installations, clay minerals and to some extent also fly ash, are suggested as suitable additives. During combustion experiments with MBM and RDF, Na and K are found to be enriched in the bed material coatings. The Ca/(Na+K) ratio found in the necks between agglomerated sand bed particles was however shown to increase if kaolin or dolomite were added as additives. This effect resulted in increased agglomeration temperatures (Öhman, Nordin et al., 2003).


13

2.3.1.2 Alternative bed materials Another way of influencing the chemistry of defluidization, is to change the bed material (most often natural sand) to other minerals, thus changing the interaction and end products of the reactions between the fuel ash and the bed minerals. In Sweden, currently used bed materials are often natural sands with a lower SiO2-content, e.g. Baskarp, Rådasand. In an attempt to summarise the most important literature in this field, Table 2 gives an overview of bed material- fuel combinations and their literature references. Conclusions and findings of these reports are shortly discussed per bed material. Table 2: Overview of references describing the effect on agglomeration of the use of alternative bed materials during FBC

Bed material Fuel Composition Reference Bauxite Low-rank coal 60.7 % Al2O3 – 26.4 % Fe2O3 (Vuthaluru, Linjewile et al., 1999)

(Vuthaluru and Zhang, 2001a) Sillimanite Low-rank coal 53.3 % Al2O3 – 43.7 % SiO2 (Vuthaluru, Linjewile et al., 1999)

(Vuthaluru and Zhang, 2001a) Mullite chamotte -3 3Al2O3*2SiO2 (Zintl, 1997) Mullite Plywood

Lucerne Al6Si12O13 (8% free SiO2) (Zintl and Ljungdahl, 2004)

Kyanite Plywood Lucerne

Al2SiO5 (8% free SiO2) (Zintl and Ljungdahl, 2004)

Foundry sand Bark Olive cross

Baskarp sand coated with bentonite (Al-rich clay) layer

(De Geyter, Eriksson et al., 2005) (De Geyter, Öhman et al., 2005 (in Press))

Calcite Limestone

Low-rank coal High-alkali agricultural waste

97.9 % CaCO3 (not indicated)

(Vuthaluru, Linjewile et al., 1999) (Vuthaluru and Zhang, 2001a, 2001b) (Bapat, Kulkarni et al., 1997)

Magnesite Low-rank coal High-alkali agricultural waste

94.3 % MgCO3

(not indicated)

(Vuthaluru, Linjewile et al., 1999) (Vuthaluru and Zhang, 2001a, 2001b) (Bapat, Kulkarni et al., 1997)

Mangesiumoxide -3 Bark Olive cross Plywood Lucerne

Sintermagnesite Melt magnesite

(Zintl, 1997) (De Geyter, Eriksson et al., 2005) (De Geyter, Öhman et al., 2005 (in Press)) (Zintl and Ljungdahl, 2004)

Dolomite -3 High-alkali agricultural waste

CaO.MgO (calcinated) (not indicated)

(Zintl, 1997) (Bapat, Kulkarni et al., 1997)

Olivine -3 Alfalfa Bark Olive cross Plywood Lucerne

Mg1,6Fe2+0,4(SiO4) (Zintl, 1997) (Zintl and Ljungdahl, 2000) (De Geyter, Eriksson et al., 2005) (Zintl and Ljungdahl, 2004)

Hematite ore Pure hematite

- (alkaline solution) 39,5% Fe2O3 – 14,3% SiO2 > 99% Fe2O3

(Grubor, Oka et al., 1995)

Zirkonium sand -3

ZrSiO4 (Zintl, 1997)

Baskarp Alfalfa Quarts with 8% feldspar, 7,7% plagioclase


Råda-sand Bark Olive cross

Quarts with 15,5% feldspar, 24,8% plagioclase

(De Geyter, Eriksson et al., 2005) (De Geyter, Öhman et al., 2005 (in Press))

Plagioclase Bark Olive cross

30-50 % Albite / 70-50% anorthite

(De Geyter, Eriksson et al., 2005)

Feldspar High-alkali agricultural waste

(not indicated) (Bapat, Kulkarni et al., 1997)

K-Fledspar Bark Olive cross

Microcline (KAlSi3O8) Albite (NaAlSi3O8)

(De Geyter, Eriksson et al., 2005)

3 In order to model agglomeration mechanisms, alkali-salts, -sulphates, -carbonates and -chlorides were used, as well as one experiment with fly -ash from a biofuel-fired installation in Malå


14

Blast- furnace slag ±35% SiO2, ±34% CaO, 10-18% MgO, ± 10 % Al2O3

Plywood Plywood+ sawdust/bark -- (no ref.) Plywood mixed with wood residues, Chicken litter, Wood mixed with industrial residue or peat, Bark, Reed canary grass, Peat, Olive residue, Straw Plywood Lucerne Plywood Lucerne

GR-granule Hyttsand Hyttsten

(Nuutinen, Ollila et al., 2000) (Laitinen, Nuutinen et al., 2000) (Zintl and Ljungdahl, 2004) (Daavitsainen, Laitinen et al., 2001) (Silvennoinen, 2003) (Nuutinen, Tiainen et al., 2004) (Eklund, Brus et al., 2003) (Brus, Öhman et al., 2004) (Zintl and Ljungdahl, 2004) (Zintl and Ljungdahl, 2004)

LD-slag -- (Zintl and Ljungdahl, 2004) AGGLOSTOPTM Plywood Diabase (plagioclase, olivine,

amphibole/pyroxene) (Silvennoinen, 2003)

During combustion of low-rank coal in normal sand, enrichment of Na and S in the coating on the bed parti cles was shown. Further analysis indicated the prevalence of sulphates of Na and Ca. If calcined sillimanite and bauxite was used, coatings are dominated by Al-rich phases, together with silicates of Fe, Mg and Ca forming less sticky eutectics (Vuthaluru, Linjewile et al., 1999; Vuthaluru and Zhang, 2001a). Using calcite, the bed particle coatings were enriched in Ca and S, occurring in molten regions indicating the formation of CaSO4. In a magnesite bed, coatings were enriched in Mg, Si, Ca and S. The occurrence of Na2SO4 in the ash coating as well as crystals rich in Na and S in the pore structure of the bed material suggested that these low melting compounds lost their liquid character in the presence of Mg-rich phases, rendering them less sticky. The mechanism by which these bed materials extend the operation period prior to agglomeration is believed to be due to dilution of the low melting eutectics by Ca- and Mg-related phases (Vuthaluru and Zhang, 2001a, 2001b). (Vuthaluru and Zhang, 2001a) conclude also that alternative bed materials such as sillimanite , bauxite, calcite and magnesite have an overall positive influence on fluidization behaviour, as increased levels of Al, Ca and Mg appear to result in a less sticky ash coating. Another Al-rich clay mineral was tested by (Zintl, 1997), mullite chamotte, which did perform well in tests with different K-rich salts. Phase diagrams for the system K2O – Al2O3 – SiO2 (see Appendix) show that the total composition was far from low-melting eutectics, although this system is poorly investigated in the relevant areas. On the other hand, tests with Na-rich salts were not that successful. No theory was presented to explain this difference in behaviour. Mullite and Kyanite, both aluminiumsilicates, should form high melting compounds with alkali-rich fuels. However, during crucible tests with plywood and Lucerne, melting points as low as 700-800ºC have been observed, probably due to the occurrence of free silica (Zintl and Ljungdahl, 2004). The behaviour of foundry sand showed an expressed dependency on fuel composition, as the agglomeration risk was reduced during bark combustion, but increased for olive cross combustion in foundry sand bed. This was explained based on the ternary phase diagram for the system K2O – Al2O3 – SiO2 in appendix. The “glue” composition was characterised by a high Si and K- content compared to Al upon olive cross combustion, whereas for bark, the K- content was considerably lower. This seems consistent with the phase diagram showing a dramatic decrease in melting temperatures for high K/Al ratios (>1). Also, it is found that according to this system, efforts to prevent agglomeration should benefit from favouring a high (K2+Al2)/Si ratio. Furthermore, the phase diagram for K2O – CaO – SiO2 is characterised by an important conjugation line with a first peritectic around 1075ºC at the higher Ca/K side and a eutectic at 725ºC on the lower Ca/K side (De Geyter, Eriksson et al., 2005; De Geyter, Öhman et al., 2005 (in Press)). For MgO , fuel ash/bed material interaction mechanisms seemed to differ from other bed materials, almost no layer formation could be recognised. Agglomeration did nonetheless occur with K-rich fuels and was attributed to other mechanisms, probably induced by collision of the MgO-particles with molten ash particles (De Geyter, Eriksson et al., 2005). Different behaviour of this bed material compared to quarts-based bed materials was also reported by (Zintl, 1997; Zintl and Ljungdahl, 2004), where defluidization was


15

found to be reversible and fluidization could be restored during further temperature increase. In order to explain what happens between MgO and K2O, further equilibrium studies should be done to complete relevant phase diagrams. Also calcinated dolomite (dolomite calcinates at 900°C) was successful in withstanding defluidization together with different alkali-components (Zintl, 1997). Olivine in combination with K and Na-rich components showed defluidization tendency around the tabulated melting temperature for the respective alkali-components. Formation of an attack layer has not been investigated in this study (Zintl, 1997), but it was suggested that agglomeration was induced by salt-melt and not by reaction with the bed material. In (Zintl and Ljungdahl, 2000), laboratory test runs with Baskarp sand and olivine are reported, where the combustion of alfalfa in Baskarp sand bed resulted in agglomeration only after 15 minutes. If the bed material was changed to olivine, the combustion test lasted for 4 hours, at the same temperature of 850°C. In another study (De Geyter, Eriksson et al., 2005), the behaviour of the olivine bed in combination with Ca-rich bark fuel was not significantly different from the same fuel tested in a quarts bed. However, in case of a K-rich olive fuel, olivine had a positive influence, and showed reduced agglomeration risk compared to quarts. Also in (Zintl and Ljungdahl, 2004), olivine was reported to withstand alkali-attack rather well, and the main mechanisms was believed to be collision with melted fuel ash particles, or salt melting. (Grubor, Oka et al., 1995) reports a study on the use of hematite (both in pure form and as ore) as a bed material. Addition of an alkaline solution to a fluidized bed was used to model reaction between bed material and alkali in biomass fuels. Although the reaction products did not participate in the agglomeration mechanism, melted alkalines contributed to moderate defluidization. However, an increase of elutriation caused by severe attrition was noticed, indicating that these materials probably are not really suitable for full-scale implementation. Tests with zirconium sand showed similar behaviour to mullite chamotte as reported by (Zintl, 1997). In combination with K-rich components, weak defluidization was observed at a high temperature, although during tests with NaCl, a low defluidization temperature was observed. During experiments by (Bapat, Kulkarni et al., 1997) with high-alkali agricultural wastes in dolomite-, limestone- and magnesite – bed, it was not possible to maintain sufficient bed inventory due to the high attrition rate, that was observed to be several times higher than for sand. Feldspar showed a reasonably lower attrition rate, but increased the defluidization risk significantly compared to sand beds. In order to evaluate the effect of different minerals naturally occurring in sands used in fluidized bed combustion in northern European countries, the same plagioclase and feldspar minerals as occurring in natural sands were tested separately as bed materials with bark resp. olive cross. It could be concluded that plagioclase (labradorite) has no or a slightly positive effect on agglomeration, whereas the potassium (and sodium) content of the feldspar increased the risk for agglomeration by lowering the system eutectic. The same trend could be seen from a test with Rådasand, natural sand with relatively high content of non-quarts impurities. Enrichment of feldspar-particles in the agglomerate, and an almost complete absence of plagioclase-particles, resulted in an overall increased risk for agglomeration compared to quarts (De Geyter, Eriksson et al., 2005). Blast furnace slag is today available in Scandinavia from two companies and sold under the names Hyttsand/Hyttsten or GR-Granule. Hyttsten is an air-cooled blast furnace slag, crushed and grinded to desired granulometry, whereas hyttsand is a blast-furnace slag cooled and granulated in a water bath and has therefore less crystalline phases and more amorphous glass than hyttsten. GR-Granule was introduced in full scale installations in Finland as an alternative bed mater ial for alkali-rich fuels. During 9 months of full scale normal operation tests, no agglomeration problems were observed although the fuel ash contained up to 33% of Na2O. Only small agglomerates were formed around quartz particles that were present in the bed as an impurity from fuel or residue from the previous bed. Most other particles did not agglomerate but were coated with several thin coating layers that grew gradually as the test proceeded. Innermost, the coating layer was rich in Ca and P, while the outermost layer was mainly rich in Mg. These layers are believed to protect the particles from agglomeration (Laitinen, Nuutinen et al., 2000; Nuutinen, Ollila et al., 2000; Daavitsainen, Laitinen et al., 2001). No Mg-rich regions were observed in the agglomerates (Nuutinen, Tiainen et al., 2004). The results on agglomeration temperature were confirmed during combustion of plywood in lab-scale tests, although during Lucerne combustion, some reversible defluidization occurred (Zintl and Ljungdahl, 2004). However, (Silvennoinen, 2003) reports (although without further reference) that GR-granule has been found to be unsuitable for long- term industrial application, as full scale installation suffered from severe secondary problems such as windbox and fluidization nozzle plugging. Hyttsand has been tested on a laboratory scale as well as in full scale (Eklund, Brus et al., 2003; Brus, Öhman et al., 2004; Eklund and Öhman, 2004). The main conclusions of these reports were that for most of the tested fuels, the tendency for agglomeration was decreased and the consumption of bed material was reduced with up to 30%. Since the total costs for bed material are a combination of both purchasing of new bed material and disposal of bottom ash, the high price of the bed material might be


16

motivated: use of hyttsand resulted in a reduced bed material exchange, a wider range of combustible bio-fuels and a better combustion of difficult biofuels. However, more large scale tests in a FB-boiler should be performed prior to full-scale implementation (Eklund, Brus et al., 2003; Brus, Öhman et al., 2004; Eklund and Öhman, 2004). Hyttsten and Hyttsand were also evaluated positively as alternative bed materials during combustion test with plywood and Lucerne (Zintl and Ljungdahl, 2004). LD-slag is a waste product from LD-converter where iron ore is converted to melted steel. This slag has a high content in iron. It has been discarded from experiments testing alternative bed materials, as it catalyses NOx – formation (Zintl and Ljungdahl, 2004). The commercialised and patented AGGLOSTOPTM is produced from a by-product of rock crushing and is marketed for use in fluidized beds combusting fuels with high alkali-content. Quarts in natural sand beds is replaced with other minerals, such as minerals found in naturally occurring diabase, containing less than 1% quarts, and rich in (Ca-rich) plagioclase, olivine and/or amphibole and pyroxene. According to the producer, it has been successfully used as a bed material for the combustion of Na-rich plywood residues in Finland (Silvennoinen, 2003). 2.3.1.3 Co-combustion of fuel types with different ash characteristics

2.3.1.3.1 Biomass and coal However the fact that certain coal types during combustion cause problems with agglomeration, the use of coal during co-combustion with biomass might result in positive synergistic effects:

• Higher sulphur content (often as pyrite), which may react with alkali in biomass and form sulphates, preventing the formation of low-melting alkali-silicates;

• Coal often contains clay minerals, such as montmorillonite and kaolinite, that have been suggested as suitable additives for prevention of agglomeration;

• Coal often contains carbonate inclusions, such as calcite or dolomite, also suggested as suitable additives. (Raask, 1984). The interaction of the inorganic material in solid fuels with the combustion environment during fluidized bed combustion depends on its composition and speciation. Mineral matter in coal tends to be composed of relatively large enclosures of mineralogical material. In biomass fuel, ash forming elements are mainly organically bound, or contained in very small salt particles, which is why they are expected to be more reactive and easily available during combustion (Goblirsch, Benson et al., 1983; Öhman, 1999). The chemical fractionation of ash components in different fuels and its effect on their reactivity during combustion has been investigated by a.o. (Zevenhoven-Onderwater, Blomquist et al., 2000; Zevenhoven-Onderwater, 2001). An important interaction between the biomass and coal during cofiring is the reaction of the sulphur from the coal with the alkali species from the biomass, reducing the stickiness of superheater deposits and their chlorine content, possibly reducing the corrosion potential of deposits. Sulphates are the thermodynamically favoured form of alkali species at typical deposit temperatures. Therefore if the coal provides sufficient sulphur to react with all the available alkali, then there will be no condensed phase Cl assuming thermodynamic equilibrium. Experiments indicate that the value of the proposed fuel-S to available alkali ratio should be at least 5 to avoid significant Cl-rich deposits on superheater surfaces (Robinson, Junker et al., 2002). However, even in the bed area, coal has a demonstrated positive effect. Whether this effect is due to the sulphur content in the coal or to other mineral inclusions is not clearly identified. (Svoboda, Pohorely et al., 2003) suggested that the kaolinite in coal prevents agglomeration during co-combustion with wood during PFBC.

2.3.1.3.2 Co-combustion of different types of biomass

Fuels Weight % ratio Reference Alfalfa/Forest residues 30/70-70/30 (Zintl and Ljungdahl, 2000) Alfalfa/Peat 100/0-70/30 (Zintl and Ljungdahl, 2000) Rice straw/Demolition wood 100/0 – 0/100 (Salour, Jenkins et al., 1993) Bark/Peat Logging residue/Peat

100/0-70/30 (Lundholm, Nordin et al., 2002) (Lundholm, Nordin et al., 2005)

Logging residue/Peat Straw/Peat

80/20 (Pommer, Olofsson et al., 2005)

(Zintl and Ljungdahl, 2000) report experiments with the co-combustion of alfalfa and forest residues, showing that the time and temperature prior to agglomeration strongly depend upon the fraction of forest residue in this fuel mix. Break point for improved agglomeration behaviour is found to be >50% forest residue. Some preliminary tests with peat and alfalfa showed an improvement at >30% peat. The effect was attributed to the presence of clay weathering minerals, such as kaolin.


17

Agglomeration occurred during all experiments with rice straw alone at below 650ºC. The addition of demolition wood resulted in longer operation times, but did not avoid agglomeration at this temperature. Only with blends containing over 50% of wood, successful operation was achieved under longer time periods (Salour, Jenkins et al., 1993). Peat has been used on an industrial scale in CHP-plants since the 1980’s. Despite ongoing discussions about the fossil character of peat, there are some important advantages of co-combustion with peat. Co-combustion of woody biomass with peat has resulted in important extensions of the lifetime of superheaters and minimised the occurrence of bed agglomeration, even at low mixing ratios of 5-30% (on DM) (Burvall and Öhman, 2000; Lundholm, Nordin et al., 2002; Boström, 2003). There is however a large variation in peat composition and mechanisms seem to vary according to peat composition. Bed particle coatings are not changed with respect to their potassium concentration, whereas aluminium and/or calcium are enriched when peat is co-combusted. Another mechanism that could not be excluded was the reaction of alkali metals with sulphur to form sulphates in stead of hydroxides and chlorides. Apart from changing the melting behaviour and/or viscosity of deposits, it was suggested that this mechanism might transport sulphates away from the bed area (Lundholm, Nordin et al., 2005). In another study, XRD-analysis was used to determine the mineralogical form of the ash compounds in peat. A large part of the mineral content was found to be in amorphous form and hence potentially more reactive. Ca, S and Mg were mostly available in reactive sulphate- form. In comparison with purely woody biomass combustion, bed particle inner coatings contained less alkali and more Ca. Moreover, the amount of small particles (< 1µm) was reduced, while the amount of course particles (>1µm) as well as the HCl content in the flue gases was increased. Agglomeration temperature in quartz bed was in creased with 130°C and 170°C for combustion av 20 w-% peat together with logging residue and straw respectively (Pommer, Olofsson et al., 2005).

2.3.1.3.3 S-addition as a means of minimising agglomeration tendency

One study reports a direct positive effect of S on the agglomeration risk during combustion of straw (Lin and Dam-Johansen, 1999), where the effect of coal-ash and SO2 on agglomeration tendency of straw was established separately. Main results of this study were that the use of coal-ash as a bed material decreased the agglomeration risk significantly compared to a quartz bed. Furthermore, it was shown that also the addition of SO2 to the coal ash/straw system decreased the agglomeration tendency. It was suggested that the reaction (Eq. 7) of potassium with SO2 leads to the formation of sulphate. This reaction is favourable at low temperatures

42222 SOKOSOOK →++ Eq. 7

Since the melting temperature of K2SO4 is 1069ºC, the presence of K2SO4 increases the melting temperature of the system. The effect of Eq. 7 is however temperature-dependent, and favoured at lower temperatures (Lin and Dam-Johansen, 1999). Two other independent studies report results of SO2 addition to the primary combustion air into a BFB test rig (Nordin, Öhman et al., 1995; Zintl and Ljungdahl, 2000). The objective of both SO2 experiments was to control whether the S in coal resp. peat could be replaced by addition of SO2 to the combustion air positively influencing the agglomeration behaviour. Addition of SO2 equivalent to 1% S in the fuel extended the defluidization time with about 15 to 30 minutes, which however was regarded to be insignificant (Zintl and Ljungdahl, 2000). No effect was reported by (Nordin, Öhman et al., 1995) but a potential explanation of the positive effect of coal and peat was given in that the large total surface of the coal ash particles could function as an adsorbing area capturing fluxing elements from the biomass fuel, rather than their high S-content. The addition of elementary S to the fuel has been tested in order to reduce high-temperature corrosion due to low-melting alkali-chlorides. However, adding S to the fuel was reported to lead to an important increase in SO2-level, increasing the acidity and sulphate content of flue gas condensate at the cold end of the boiler, in its turn increasing the risk for low temperature corrosion (Henderson, Kassman et al., 2002). 2.3.2 Process control-based measures: early warning systems Despite efforts to understand the mechanisms and attempts to interfere with the cause of agglomeration problems, advanced process control tools could be most helpful when using “difficult” biomasses on an industr ial scale. A method based on the use of short- term predictability of pressure fluctuations was proposed by (Schouten and van den Bleek, 1998) and subsequently developed by (van Ommen, Schouten et al., 1999b; van Ommen, Schouten et al., 1999a; van Ommen, Schouten et al., 2001a; van Ommen, Schouten et al., 2001b; Korbee, van Ommen et al., 2003) It is based on a comparison of a reference time-series of pressure fluctuations with a time-series taken during operation and is also known as Early Agglomeration Recognition System (EARS). The mathematical technique used is called “attractor reconstruction”, where the attractor is a multi -dimensional distribution of delay vectors, containing consecutive pressure measurements. Reference attractor and evaluation attractor are compared by calculating a statistic S, representing the dimensionless distance between the two attractors. The reaction of the S-statistic has been shown to appear before any deviations in temperature and pressure can be observed (Korbee, van Ommen et al., 2003)


18

Another method uses multivariate process data to calculate a reference. Subsequent process data are then evaluated with respect to their distance to this model, a procedure known as Principal Components Analysis (PCA). Variations in pressure and CO concentrations in the bed were found to react first and approximately 3 hours prior to what was defined to severe abnormalities in the process behaviour. Further refining of this model might allow industrial application (Öhman, Hokfors et al., 2002). Control of agglomeration can also be tested by a rotating furnace device as proposed by (Berge, 2005). By following the development of a sintring index of bed sand sample taken each day in a rotating furnace with video camera equipped with automated image analysis, the lowering of the agglomeration temperature in the bed could be followed until the bed agglomerated. This technique requires however sampling of bed material and some manual operations.


19

3 A practical example: the Idbäcken plant

3.1 History The plant was built in Nyköping in 1984, at that time comprising of two boilers (P1 and P2) with coal as the main fuel. In 1994, the Idbäcken plant was enlarged by the addition of third line (P3), for combined heat and power generation. Vattenfall AB bought the plant in 1996 from the municipality of Nyköping. In 1996, the fuel mix consisted of 15% coal and 85% wood fuel (forest residues, bark, sawdust and up to 20% waste wood). Over the years however, increasingly contaminated fuels have been incinerated. During 1998, coal was replaced by plastic waste for half a season. In 2000 the wood fraction consisted of equal shares waste wood and “clean” wood, and 15% peat. Due to ash related problems, however, the fuel mix was changed to more expensive and less contaminated wood shares, with a maximum of 20% waste wood of a better quality. Until autumn 2003, the Idbäcken plant used a fuel mix consisting of 85-88% biomass (waste wood and forest residues) and 12-15% coal (% on energy basis). Coal was however removed from the process because of economic reasons. Today, the fuel mix consists of 50-75% waste wood and 50-25% forest residues. During 2001-2004, mainly fouling and corrosion problems have been tackled during the “REHAB” project. Although good results on fouling and corrosion, at high capacity there still is a risk for high sand consumption and even bed agglomeration.

3.2 Technical data 3.2.1 P1 and P2 The oldest parts of the installation consist of P1 and P2, 2 identical circulating fluidized bed (CFB) furnaces, today used only during the summer and during periods of peak heat demands in wintertime. 3.2.2 P3 P3 is a furnace of the bubbling fluidized bed (BFB) type, designed by the Finish company Outokumpu Ecoenergy OY. An overview of the installed thermal capacity is given in Table 3. Table 3: Thermal capacity of Idbäcken plant

Thermal capacity MW Heat Electricity

70 (+15 flue gas condensation)) 35

According to (Andersson, Andersson et al.) Steam is produced at 40 kg/s, 140 bar and 540°C. Flue gas condensing is used to further increase thermal efficiency of the plant. The components of the installation are illustrated in Figure 7.


20

Figure 7: Overview of P3 process equipment

Fuel feeding into the 3 boilers is organised from a large fuel storage area, where wood chips from different origins are sorted into different piles. The wood chips are transported and fed into a fuel hopper by means of a wheel loader with a maximum of 12 m3 loading capacity. Following assortments of fuel are regularly available at site:

• Imported and domestic chipped forestry residues or sawmill residues

• Chipped demolition wood: domestic or imported from European countries, classed as non-dangerous wood waste (with limited amounts of paint and no wood preservatives)

The control room is in regular contact with the operator of the wheel loader to give instructions with regards to the required fuel mix. Usually th e operator is instructed to supply the plant with equal volumes of the three assortments above. Ideally, these assortments are premixed in a dedicated stack on the storage site, prior to loading into the fuel hopper. At periods of high demand however, time is too limited for all mixing operations to be successfully completed, and the fuel pretreatment and transport train has to be counted on for mixing of the different fuel assortments. From the hopper, the fuel is transported by means of a chain conveyor towards a pair of magnetic separators, automatically removing metallic contaminations. A vertical bucket elevator then transports the fuel towards two lines of injection into the P3 furnace. Each line is equipped with a small fuel buffer covering operation at full load during approximately 30 min. The buffer is emptied by a frequency controlled extraction screw, placed below a set of bridge breakers and a permanent level regulator. This screw determines the fuel flow towards the furnace. The residual pathway between fuel hopper extraction screw and furnace consists of a set of chain conveyors. The fuel flow of each line is 4 kg/sec during high load. Fuel is added to the furnace from two different lines. Primary combustion air is preheated to 200°C by heat exchangers in countercurrent in the fluegases leaving the last economiser. The primary air consists partly of recirculated flue gases taken from the chimney inlet and is used as fluidization air. Secondary air enters the furnace at several levels higher up. The last level of air inlet is the over fire air (OFA) used in order to ensure complete burnout of the fluegases prior to reaching the superheaters. The sand bed (65 ton) is kept at a temperature of about 850°C while the flue gases reach temperatures above 1000°C. Temperature profiles of the flue gases have been improved considerably during the REHAB project, by changing the inlet flow profiles of secondary and OFA air. The flue gases pass subsequently through three superheaters, one of which is in verti cal position and three economisers. The NOx catalyst shown in Figure 7 has been removed. Prior to entering into the electrostatic precipitator (ESP), the flue gases are cooled down by air preheaters. After the ESP, the flue gases are further cooled down by the flue gas condensor to about 60°C prior to entering the ID-fan and the chimney.


21

At the first superheater, heavy particulates in the flue gases are sedimented and drained down to the second pass. Bottom ash and spent bed material are taken out and stored in an ash silo for transport to landfill. Fly ash from the ESP is collected separately in an ash silo.

3.3 Ash related problems at the Idbäcken plant The main challenge for the Idbäcken plant after the successful conclusions of the REHAB project was to operate on biomass only, without coal. It is estimated that this would improve the economy of the plant by 0,5 million EUR/year (Andersson, Andersson et al.). During the seasons 2004-2005, this was done rather successfully. Some problems were caused by heavy contaminants in some of the supplies of demolition wood, containing large amounts of metal scrap, nails etc. These can impair the fluidisation and thereby cause agglomeration. Slagging on the walls of the furnace was alleviated by the installation of an extra furnace control camera. Injection of water on the furnace walls was applied to break the slag in time, before large lumps could fall down into the bed and impair fluidisation.


22

4 Materials and method Some dedicated bench-scale experiments were done to evaluate the effect of sulphur on the agglomeration characteristics of typical Idbäcken fuel as well as some other typical biomasses representing different agglomeration mechanisms according to (Brus, Öhman et al., 2005). Combustion experiments were done in a bench-scale fluidized bed reactor with a range of different sulphur-based additives in combination with the different fuel types (bark and olive residue). Thermo-chemical equilibrium calculations were used in order to determine the theoretical level of sulphur addition necessary for minimising the formation of liquid phases in the reaction products which are important for the agglomeration process. For all fuel/additive combinations, the agglomeration temperature was defined and bed material samples were collected and analysed using Scanning Electron Microscopy and Energy Dispersive X-Ray analysis (SEM/EDS). During the combustion experiments, the chemical composition of the reaction atmosphere was measured using a Fourrier-Transform Infra Red gas analysis setup (FTIR). Results of the SEM/EDS analysis were subjected to further thermo-chemical equilibrium calculations in order to require a better understanding of the possible mechanisms involved.

4.1 Hypotheses at the basis of the experiments Following arguments for evaluating the effect of S on bed agglomeration could be distilled from the literature study:

• The reaction of K in the fuel with SiO2 of the bed particles resulting in low-melting alkali - silicates is one of the main starting points for agglomeration during combustion of alkali-rich biomass fuel (Brus, Öhman et al., 2005)

• The introduction of S into bed area might shift the reaction equilibrium towards the high-melting K2SO4, thereby reducing the availability of K for the formation of low-melting silicates and the risk of agglomeration (Lin and Dam-Johansen, 1999)

• Alkali-sulphates could in this way be elutriated up and out of the bed area without interaction with the bed material, thereby preventing agglomeration (as suggested by (Lundholm, Nordin et al., 2005)

• Risk of agglomeration during FBC of high-S fuels is reported to be due mainly to the particle characteristics of limestone or dolomite additives, rather than to the presence of S itself. The results of these studies are therefore not interfering with the logic of the experiments (Anthony, Iribarne et al., 1995)

Agglomeration risk based on low melting alkali-sulphates exists when high concentrations of Na prevail (Goblirsch, Benson et al., 1983). Addition of S to these fuels might therefore not work. The Idbäcken fuel showed however rather low Na-concentrations.

4.2 Thermo-chemical equilibrium calculations Prior to bench-scale experiments, FactSage 5.4 was used to evaluate which levels of S were necessary in terms of chemical equilibrium in order to influence melt formation and thereby the agglomeration characteristics. The elements and databases used in the calculations are listed in Table 4. Table 4: Input elements and solution models used in the chemical equilibrium model calculations

Input elements: H, C, O, N, Cl, H2O, O2, N2, Si, Al, Fe, Ca, Mg, Na, K, Mn, P, S

Solution models: Components

Description, FactSage 5.4. name

SLAG SALT

MgO, FeO, MnO, Na2O, SiO2, CaO, Al2O3, K2O, Fe2O3 NaCl, KCl, NaOH, KOH, Na2SO4, K2SO4, Na2CO3, K2CO3, NaNO3, KNO3 Na,K//SO4,CO3 MgSO4-CaSO4 K,(Ca)//CO3, SO4 Na, (Mg,Ca)//SO4 K, Ca//CO3,SO4 Ca, Mg, Na//SO4

FToxid – FACT oxide liquid solutions (Aug. 2005) - ?Slag-liq FTsalt – Fact salt solutions (Aug. 2005) Liquid solutions – (FSalt-SALTF) Solid solutions (FTSalt-SCOB) Solid solutions (FTSalt-SCMO) Solid solutions (FTSalt-SCSO) Solid solutions (FTSalt-SSUL) Liquid solutions (FTSalt-LCSO) Liquid solutions (FTSalt-LSUL)

The software program uses minimization of the total Gibbs free energy of the system to calculate concentration and components at chemical equilibrium. All binary solid and liquid solutions with Ca, Mg, Na and K were included as well as two separate liquid phases, comprising the potentially coexisting oxide/silicate (slag) melt and alkali salt melt.


23

The chemical equilibrium was calculated based on the following assumptions:

• Only the outer 10µm of all the bed particles participate in reactions with the fuel ash

• The total amount of fuel added corresponds to a combustion with air factor 1,4

• The equilibrium calculation temperature was 800ºC for bark and 760ºC for olive residues Results are displayed as graphs with S-equivalents in the x-axis. 1 S-equivalent = sum (Na2+K2+Ca+Mg) in fuel ash. The y-axis displays – in a logarithmic scale – the total amount of mole at equilibrium of some interesting gases, slag and solid phases. Elemental SEM/EDS analysis results were further analysed using FactSage 5.4: melting behaviour and main equilibrium shifts during sulphur addition. Melting behaviour was calculated on the agglomerate neck compositions of the samples. It gives the variation of the weight-percentage of molten phases in function of the temperature. This allows evaluating whether the theoretical melting behaviour corresponds to the agglomeration characteristics from the bench-scale experiments. Input elements for this calculation are the elemental analysis in combination with the main components of the relevant combustion atmosphere, calculated from the fuel composition and air factor. Solutions models are the same as in Table 4. Main equilibrium shifts during sulphur addition were studied using Ca, K, Si- concentrations found in SEM/EDS of outer layers of the bed particles. Input elements for this calculation are the elemental concentrations of Ca, K and Si in the outer layers in combination with the main components of the relevant combustion atmosphere and varying concentrations of sulphur. Solutions models are the same as in Table 4, as far as they contain the input elements. In order to evaluate to what extent liquid sulphates might be stable, the Slag-model was however changed to FToxid – SLAGB, that except for oxides even includes liquid sulphate phases.


24

4.3 Bench-scale combustion experiments 4.3.1 The CFBA-method The Controlled Fluidized Bed Agglomeration method (CFBA), described in detail by (Öhman and Nordin, 1998), was used for combustion and determination of the agglomeration tendencies. The reactor consists of a cylindrical bed and freeboard section constructed in stainless steel with a diameter of 100 and 200 mm respectively. Electrical wall heaters provide isothermal conditions minimizing the influence of cold wall effects.

Prim.Air

PropaneSec.Air

Pre-heater

Wall heater

F1

F2 F3

Propane burner

Fuel

Pump

CO

CO2

O2

NO

THC

Cyclone

Ventilation

Condenser

T6

T7

T8

T5

T4

T3

P4

T2

P3

T1P2

P1

T/P Signals

Data Acquisition System with On-Line PCA

F4

.

.

.

.

. .

.

..

.

DP

.

View window

.

x

Figure 8: Bench-scale fluidized bed reactor ; P1-P4: differential pressures, T1-T8: thermocouples, F1-F3: mass flow controllers, DP: distributor plate

The agglomeration was initiated by normal fluidized bed combustion. The oxygen content in the flue gases was maintained at 6% and the combustion temperature was kept at 800º C for bark and 760ºC for olive residue in order to minimise the risk for direct agglomeration. The air flow was set at 80 l/min, corresponding to 8 times the minimum fluidization velocity. At an ash amount theoretically corresponding to 20 wt-% on bed material, the fuel feeding was stopped to avoid the uncertainty of the burning particle temperature and external heating was switched on. In order to maintain a combustion atmosphere in the reactor during the heating phase, propane was burnt with primary air in a combustion chamber under the distribution plate and the fluidization velocity was reduced to 4 times the minimum fluidization velocity. The bed was heated continuously and isothermally at 3ºC/min until bed agglomeration was achieved or until 1050ºC was reached. The onset of defluidization was indicated by deviating differential pressures and temperatures in the bed. This methodology has been evaluated to give a reproducibility of ±5ºC (SD) (Öhman and Nordin, 1998). The formation of sulphates is however highly dependent on the oxidative character of the environment. The bench-scale experiments according to the CFBA method are usually done with primary air only. Since the air injection in full-scale installation is distributed over primary, secondary and often even tertiary injection points, the effect of staged combustion is evaluated in a separate experiment where 50% of the primary air is replaced by N2 and the remaining 50% injected above the bed area as secondary air. Apart from experiments with fuel combustion, the last part of the CFBA-method was also used to evaluate the agglomeration characteristics of full- scale bed material samples taken from the Idbäcken plant. In this case, 542 g of bed material was added to the reactor and heated to 800ºC. Then, the minimum fluidization velocity was determined and the experiment was continued at several


25

times the fluidization velocity. The bed was heated externally in a propane combustion atmosphere at 3ºC/min during until bed agglomeration was achieved or until 1050ºC was reached. 4.3.2 Fuels In the Idbäcken plant, 50-75 % of the incoming biomass fuel consists of demolition wood chips. Demolition wood can in principle contain all kinds of contaminants, which can be classified either as chemical or as mechanical contaminations. The latter are defined as more or less easily separable from the biomass material. Chemical contaminants however are bound to the biomass matrix and almost impossible to remove (Jermer, Ekvall et al., 2001). A representative sample of Idbäcken fuel (September 2004) was pelletised and tested for its agglomeration characteristics separately as well as with coal in order to demonstrate the effect of coal co-combustion on this specific fuel mix. In order to get an understanding of how the addition of sulphur might influence the previously identified agglomeration mechanisms, some model fuels were selected that have well-documented agglomeration mechanisms in a quartz bed: bark and olive residue are typical for the most current agglomeration mechanisms identified for biomass combustion (see paragraph 2.2.1). Calcium-rich bark is regarded as a model fuel for woody fuels typically following agglomeration mechanism (a) and olive residue is model for potassium-rich agro-fuels, typically following mechanism (b). Bark and olive residue were used together with different forms and levels of sulphur addition. Fuel compositions are summarised in Table 5. For reference, the median composition of demolition wood as reported by (Strömberg, 2005) is shown as well. Idbäcken fuel is shown to have much lower sodium content than average demolition wood. Zinc is also a component that normally is enriched in demolition wood. It was however not measured in the tested Idbäcken fuel. SEM/EDS analysis did not show any traces of Zinc enrichment in coatings or agglomerate necks.

Table 5: Fuel composition (wt % on D.S.)

Bark Olive flesh Idbäcken fuel Average demolition wood for ref.

(Strömberg, 2005)

Coal

Dry Substance (D.S.)* 93,7 85,1 86,5 94,1 Ash content 3,6 6,6 3,6 5,8 12,2

C 41,6 52,04 49,8 48,9 74,3 H 5,9 6,53 6,3 5,9 4,5 O 48,1 33,28 39,64 37.9 6,52 N 0,8 1,45 0,6 1,1 1,8 S 0,03 0,0924 0,05 0,075 0,62 Cl 0,01 0,099 0,04 0,052 0,03

SiO2 0,735 1,06 1,68 1,263 Al2O3 0,109 0,172 0,284 0,592 CaO 1,28 0,896 0,674 0,55

Fe2O3 0,0582 0,121 0,133 0,374 K2O 0,253 2,18 0,164 0,287 MgO 0,122 0,34 0,131 0,113 MnO 0,0572 0,003 0,028 0,013

Na2O 0,0324 0,044 0,066 0,225 P2O5 0,086 0,287 0,11 0,048 TiO2 0,0044 0,009 0,078 0,171

* wt-% on fuel

4.4 Additives and overview of experiments Coal was added to the pelletised Idbäcken fuel in a 90/10 ratio (w-%) by bulk fuel mixing of sieved coal particles. Sulphur was added in different forms to the combustion zone, together with the fuel or with the primary combustion air. Elementary S, SO2 and (NH4)2SO4 were dosed at levels in accordance with the results of the thermo-chemical equilibrium calculations. Elementary S (in small spherical granule form) was mixed in bulk into the pellets, SO2 was mixed into the primary


26

combustion air as 3% (v-%) gas and (NH4)2SO4 was dissolved in demineralised water and distributed evenly by spraying over the pellets, which were then left to dry over night. Table 6: Overview of the performed bench-scale experiments

No additive Coal Elementary

S SO2 (NH4)2SO4

Idbäcken fuel x 90/10 (w%) biomass/coal

- - -

Bark x - 12.3 gS/kg fuel

- -

Fuel combustion

Olive residues x - 1) 9.7 gS/kg fuel

2) 9.7+ gS/kg fuel

1) 9.7 gS/kg fuel

2) 1.2* gS/kg fuel

9.7 gS/kg fuel

100% biomass x - - 4.5 g S/kg bed material

- Full-scale bed material

90/10 (w%) biomass/coal

x - - - -

x = experiment performed ; - = not performed + = experiment also performed with 50% primary and 50 % secondary air, in stead of 100%¤ primary air. In order to obtain the same fluidization conditions in the bed area as for the other experiments, N2-gas was added to the primary air flow. * = olive residue was tested with low S-addition, close to an acceptable level for real-scale installations Apart from experiments with a combustion phase, bed material samples taken at the Idbäcken plant during a period with and without coal addition were tested for their agglomeration characteristics. A suggestion of VUAB to treat bed material with sulphur during the intermediate storage prior to recirculation to the bed (although Idbäcken is a bubbling bed, a small recirculation of bed material is applied). In order to test this, a bed material sample from a period without coal combustion was heated and subjected to a SO2 flow, added with the fluidization air during 30 minutes prior to the temperature ramping phase. The total amount added to the bed material in th e reactor corresponded to 4,5 g S / kg bed material.

4.5 Flue gas analysis A Fourrier-Transform Infrared flue gas analysis instrument (FTIR) was used to analyse flue gases sampled at about 10 cm above the bubbling bed. The sampled gases were taken at a temperature of about 650ºC and kept at a temperature above 200ºC prior to entrance into the ceramic filter of the FTIR sampling facility. The results of the flue gas analysis were analysed logged with 30 sec. intervals. Compounds measured were H2O, CO2, SO2, NO2, CO, NO, HCl, NH3, N2O and CH4.

4.6 SEM/EDS analysis of bed material Bed samples before agglomeration and agglomerates were mounted in epoxy, cross-sectioned and polished. Scanning Electron Microscopy was used to analyze the material, combined with energy dispersive X-ray spectroscopy (EDS) in order to determine the elemental composition of necks between agglomerated particles, bed particle coating and attack layers. 4-5 particles were analysed for each sample. 4-5 spots evenly distributed over the particle’s periphery were used for elementary analysis. Thickness of the coating layers was evaluated qualitatively. For agglomerate necks, 20-40 spots were analysed.


27

Figure 9: Bed particles mounted in epoxy for SEM/EDS analysis

Epoxy

Bed particle in cross-section


28

5 Results

5.1 Thermo-chemical equilibrium calculations The results of the thermo-chemical equilibrium calculations are given in Figure 10 and Figure 11.

Mol-SO2(g)

Mol-SO3(g)

Mol-HCl(g)

Mol-NaCl(g)

Mol-KOH(g)

Mol-KCl(g)

Mol-Na2O(d-SLAG?)

Mol-SiO2(d-SLAG?)

Mol-CaO(d-SLAG?)

Mol-Al2O3(d-SLAG?)

Mol-K2O(d-SLAG?)

Mol-Na2SO4(t-CSOB)

Mol-K2SO4(t-CSOB)

Mol-CaSO4(t-SCMO)

Mol-MgSO4(t-SCMO)

Mol-SiO2(s2)

Mol-MgSiO3(s2)

Mol-KAlSi2O6(s2)

Mol-CaSiO3(s)

Mol-CaMgSi2O6(s)

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

100 0.5 1 1.5 2 2.5 3

S - equivalents (Na2 + K2 + Ca+ Mg)

mo

le

Mol-SO2(g)

Mol-SO3(g)

Mol-HCl(g)

Mol-NaCl(g)

Mol-KOH(g)

Mol-KCl(g)

Mol-Na2O(d-SLAG?)

Mol-SiO2(d-SLAG?)

Mol-CaO(d-SLAG?)

Mol-Al2O3(d-SLAG?)

Mol-K2O(d-SLAG?)

Mol-Na2CO3(t-CSOB)

Mol-K2CO3(t-CSOB)

Mol-Na2SO4(t-CSOB)

Mol-K2SO4(t-CSOB)

Mol-CaSO4(t-SCMO)

Mol-MgSO4(t-SCMO)

Mol-SiO2(s2)

Mol-MgSiO3(s2)

Mol-KAlSi2O6(s2)

Mol-CaSiO3(s)

Mol-CaMgSi2O6(s)

Figure 10: Result of thermo-chemical equilibrium calculations for addition of a varying amount of S during the combustion of bark. Combustion temperature 800ºC ; 10µm of the quartz bed particles is assumed to take part in the reaction ; air factor 1,4 ; fuel ash/bed material = 20%. 1 S-eq. = 0,272 mol S/kg fuel

For comb ustion of Bark, 1 S-equivalent corresponds to 0,272 mol S/kg fuel. The model calculation assumes that no S is available at origin. In reality, the fuel contains 0,009 mol S/kg or 0,03 S-eq. For reference, the industrial acceptable level of S-addition is about 60 mg/MJ (Vattenfall). Together with the fuel-S, this level corresponds to 0,15 S-eq. Figure 10 shows however rather stable SLAG-phases until a S-addition of about 1,45 S-eq (marked with the red vertical line). At this point, alkali and alkaline earth elements in the liquid slag-phases disappear from the equilibrium and are found to form mainly solid sulphates. Part of the potassium is also forming KAlSi2O6 (leucite). The rest of the Si is forming SiO2. This level (1,45 S-eq or 12.3 g/kg fuel) was therefore identified as set point for further bench-scale experiments. This is about 12 times higher than what would be industrially acceptable.


29

Mol-SO2(g)

Mol-SO3(g)

Mol-Cl(g)

Mol-Cl2(g)

Mol-HCl(g)

Mol-NaCl(g)

Mol-KOH(g)

Mol-KCl(g)

Mol-(KCl)2(g)

Mol-Na2O(d-SLAG?)

Mol-SiO2(d-SLAG?)

Mol-K2O(d-SLAG?)

Mol-Na2SO4(t-CSOB)

Mol-K2SO4(t-CSOB)Mol-CaSO4(t-SCMO)

Mol-MgSO4(t-SCMO)

Mol-SiO2(s2)

Mol-MgSiO3(s2)Mol-KAlSi2O6(s2)

Mol-CaMgSi2O6(s)

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

100 0.5 1 1.5 2 2.5 3

S - equivalents (Na2 + K2 + Ca + Mg)

mo

leMol-SO2(g)

Mol-SO3(g)

Mol-Cl(g)

Mol-Cl2(g)

Mol-HCl(g)

Mol-NaCl(g)

Mol-KOH(g)

Mol-KCl(g)

Mol-(KCl)2(g)

Mol-Na2O(d-SLAG?)

Mol-SiO2(d-SLAG?)

Mol-CaO(d-SLAG?)

Mol-Al2O3(d-SLAG?)

Mol-K2O(d-SLAG?)

Mol-Na2SO4(t-CSOB)

Mol-K2SO4(t-CSOB)

Mol-CaSO4(t-SCMO)

Mol-MgSO4(t-SCMO)

Mol-SiO2(s2)

Mol-MgSiO3(s2)

Mol-KAlSi2O6(s2)

Mol-CaSiO3(s)

Mol-CaMgSi2O6(s)

Figure 11: Result of the thermo-chemical equilibrium calculations for addition of a varying amount of S during the combustion of olive residue. Combustion temperature 760ºC ; 10µm of the quartz bed particles is assumed to take part in the reaction ; air factor 1,4 ; fuel ash/bed material = 20%. 1 S-eq. = 0,411 mol S/kg fuel

For olive residue (Figure 11), 1 S-equivalent corresponds to 0,4107 mol S/kg fuel. The fuel contains already 0,03 mol S/kg, or 0,073 S-eq. In this case, the acceptable level for industrial addition of sulphur, 60 mgS/MJ, corresponds to 0,133 S-eq (together with fuel-S). However, according to Figure 11, the liquid Slag-phases disappear from the resulting equilibrium at around 0,8 S-eq, (marked with a vertical red line). Beyond this line, solid sulphates are formed, and potassium forms leucite according to the equilibrium calculations. The level for bench-scale testing of S-addition was therefore chosen at 0,8 S-eq (or 9.7 g S/kg fuel). This is about 10 times higher than what would be industrially acceptable. Some problematic compounds in the gas phase are found to decrease with increasing S-content in the fuel: KCl, KOH, NaCl. The concentration of HCl is mainly influenced at low levels of S-addition (0-0,3 S-eq.).


30

5.2 Agglomeration temperatures Two types of bench-scale experiments were performed as can be seen from Table 6: agglomeration upon fuel combustion in quartz bed and agglomeration of bed material samples from the Idbäcken-plant (without fuel combustion in the laboratory). The results of the agglomeration temperatures are shown in Figure 12 and Figure 13 respectively.

800 850 900 950 1000 1050

Bark - no additive

Bark - no additive (*)

Bark - elementary S

Olive - no additive

Olive - no additive (+)

Olive - elementary S

Olive - elementary S + sec air

Olive - SO2

Olive - low SO2

Olive - Am sulph.

Idbäcken - no additive

Idbäcken - coal

Temperature

Figure 12: Overview of the agglomeration temperatures (in ºC) for the different fuel and additive combinations (*) replica of the experiment bark without additive ; (+) replica of the experiment olive residue without additive taken from (De Geyter, Eriksson et al., 2005), where the primary air flow however was 4 times the minimum fluidization velocity during fuel combustion in stead of 8 times in the other experiments

The combustion of Idbäcken fuel was done with and without coal addition. The agglomeration temperature of their ordinary fuel mix without coal addition was over 1020ºC. This is consistent with the late results at full scale: the plant has not used any coal addition during the season 2004-2006. Addition of coal at 90/10 (biofuel/coal, w-%) did further increase the agglomeration temperature according to the CFBA-method. Bark was combusted separately and in combination with the predetermined level of elementary S. No effect of S-addition was seen on the agglomeration characteristics. Therefore, further experiments with bark and other forms of S-addition were cancelled. For olive residue however, an important increase of the agglomeration temperature was found with all forms of S-addition. The effect of distribution of the air input over primary and secondary air did not influence the agglomeration behaviour significantly. SO2 gas was slightly less active than the other S-additives. At more realistic levels of S-addition (around 60 mg/MJ), the effect of S-addition was much reduced.


31

800 850 900 950 1000 1050

Idbäcken Bed 20040107 -80/20

Idbäcken Bed 20041116 - 100

Idbäcken Bed 20041116 - 100+ SO2

Temperature

Figure 13: Overview of the agglomeration temperatures (in ºC) for the bed material samples of the Idbäcken plant

The agglomeration characteristics of bed samples from the Idbäcken plant (Figure 13) reveal significant differences between samples upon coal addition (20040107) and samples without coal addition (20041116). Bed material from a period without coal addition was treated in a separate experiment, by injection of SO2 gas (4,5 g S/kg bed) to the fluidization air during 30 minutes prior to the temperature ramping stage of the experiment. However, no effect could be seen on the agglomeration characteristics.

5.3 Flue gas analyses

0

100

200

300

400

500

600

700

800

SO2 HCl

Bark - no additiveBark - elementary SOlive residue - no additiveOlive residue - elem SOlive residue - elementary S + sec airOlive residue - SO2Olive - Am. Sulph.Idb. fuel - no additiveIdb. fuel - coal

Figure 14: FTIR-flue gas analyses results (ppm) for SO2 and HCl sampled just above the bed area, corrected for 11% O2, (dry)


32

When Idbäcken fuel was used with addition of coal, a tendency of a higher SO2 – content in the flue gases just above the bed area was observed, HCl was not influenced by coal addition. For bark, the addition of elementary S to the fuel had a clear effect on the SO2 – content of the flue gases. When olive residue was combusted without addition of any additive, the concentration of SO2 and HCl was very low above the bed area. All forms of sulphur addition except for ammonium sulphate, resulted in significant increases for both SO2 and HCl. During the experiment with low SO2-addition, the FTIR instrument failed due to a power failure.

5.4 SEM/EDS-analyses The results of the elemental analyses are shown in Figure 15, Figure 16 and Figure 17. Differences in morphology between treated and untreated samples were not analysed more in detail. Examples for all fuels and treatments are shown with SEM pictures in Appendix. Generally, the layer thickness differed between the fuels. Olive residue resulted in thicker layers than bark and Idbäcken fuel. Thickness of the attack and coating layers was not influenced significantly by sulphur addition.

0

10

20

30

40

50

60

70

80

90

100

Si P S K Ca

Bark - no additiveBark - Elementary SOlive residue - no additiveOlive residue - Elementary SOlive residue - Elementary S + sec airOlive residue - SO2Olive residue - low SO2Olive residue - Am. Sulph.Idb. fuel - no additiveIdb. fuel - coal

Figure 15: Results of SEM- analyses of the inner attack layers on bed particles taken from the CFBA-experiment prior to the external heating phase

The results of the SEM-analyses of the inner attack layers revealed almost no significant differences between the different fuels and treatments. For bark combustion, a tendency for sulphur enrichment in the inner layer was seen upon addition of elementary sulphur to the combustion area. The difference was however not significant. Inner attack layers for the combustion experiments with bark were mainly characterised by Si, K and Ca. Samples taken from the experiments with Idbäcken fuel showed almost no attack layer formation. Analyses of inner and outer layer were therefore difficult. The results show a high Si-content in the inner layer, although a clear enrichment in both Ca en S can be seen for the experiment where 10 w% coal was added. No significant differences were found between the inner layer compositions of the different experiments using olive residue as a fuel. Some tendency for higher S-contents however can be seen for the treatments with elementary sulphur and SO2. The treatment with ammonium sulphate did not result in sulphur-enrichment of the inner attack layer. Potassium-contents are similar for treated and untreated samples.


33

0

10

20

30

40

50

60

70

80

90

100

Si P S K Ca


Figure 16: Results of SEM-analyses of the outer attack layers on bed particles taken from the CFBA-experiment prior to the external heating phase

The Si-content of the outer layers is reduced and higher concentrations fuel ash-related elements are found in comparison to the inner layer analyses results. However, as the spatial resolution for SEM/EDS quantification is only a few micrometers, the influence of bed material or from different attack layers can not be totally excluded. As can be seen from Figure 15 and Figure 16, inner layer compositions are situated quite exactly in between pure quartz and outer layer compositions, which might also explain the lower level of significance found in the inner layer elemental compositions. For further discussions, only the outer layer results will be used, assuming that there is no actual difference between inner and outer layer. For combustion of bark, the Si-content was significantly lower in the outer coating layer for the samples taken upon sulphur-addition and S was significantly enriched compared to the untreated sample. Addition of coal to the Idbäcken fuel resulted in significantly lower Si-content in the outer coating layer of the bed particles. Ca, S and K are however enriched. The differences between the treatments of olive residue with different forms of sulphur were not significant. The untreated sample as well as the sample from low-SO2 addition contained significantly less sulphur in the outer coating layer than the treated olive residue samples. Potassium contents are similar for the treated and untreated samples.


34

0

10

20

30

40

50

60

70

80

90

100

Si P S K Ca


Figure 17: Results of SEM-analyses of the agglomerate necks between bed particles taken from the CFBA-experiment upon agglomeration

The conclusions for the analysis of agglomerate necks are much the same as for the outer coating layer. For bark combustion, the Si concentration is lower and sulphur is enriched in the agglomerate necks of the samples with elementary S in the combustion area compared to pure bark combustion. A non-significant tendency of increased Ca-content in the treated samples is seen. Agglomerate necks of Idbäcken fuel with coal combustion are enriched in S and Ca, whereas the Si-content is significantly lower compared to the experiment without coal. More K is also found in the agglomerate necks with coal, but the difference is not significant. For olive residue combustion, the Si-content of the agglomerate necks differs between the untreated and low-SO2 sample compared to the SO2-treatment which has significantly lower Si-content. Other S- treatments did not result in significant differences in Si-content. Sulphur is enriched in the agglomerate necks of all the samples with sulphur treatment. Potassium-contents are similar in the necks of treated and untreated samples.


35

6 Discussion 6.1 Idbäcken and coal addition A representative sample of biofuel used at the Idbäcken plant was found to result in a rather high initial agglomeration temperature (over 1020ºC) using the CFBA method. This result was in agreement with the ash analysis of the test fuel, showing only low levels of chemical contaminants. The fuel was similar in composition to uncontaminated woody fuel. When coal was added to the Idbäcken fuel, a nevertheless important increase in agglomeration temperature was found. From SEM-analyses it could be concluded that coal addition resulted in significantly higher levels of Ca and S in the bed material coatings. It could not be concluded whether Ca was enriched by reaction with the higher S-content of the coal or whether it originated from the coal ash. Similar trends – however not significant – are seen from the experiment in which bark was combusted with and without S-addition. Full-scale bed material samples from the Idbäcken plant resulted in a lower agglomeration temperature (around 930ºC), which in normal operation still is above the range of normal bed temperatures. The difference with the results of the CFBA method including fuel comb ustion is most probably caused by inevitable fuel variations within the same season. Treatment with SO2 of a full-scale bed material sample from a period without coal addition did not have any effect on the agglomeration temperature. It is therefore likely that interaction between sulphur, fuel ash elements and bed material takes place during fuel combustion.

6.2 Sulphur addition to model fuels Formation of some sort of a liquid phase is according to the literature study a generally acknowledged starting point for the agglomeration process. Apart from the occurrence of a liquid or molten phase in the bed area, other parameters such as viscosity and the amount of molten material will also play an important role. Thermo-chemical equilibrium calculations supported the hypothesis that addition of sulphur to the combustion zone might influence the formation of liquid phases. For both model fuels, bark and olive cross, the liquid slag compounds were found to disappear if only sufficient sulphur was added, mainly due to a gradual sulphatisation of the alkali and alkaline earth compounds in the fuel ash. CFBA-experiments with high sulphur content resulted in significantly increased agglomeration temperatures for olive residue when sulphur was added in different forms. Lowering the amount of sulphur added to an industrially acceptable level was not found to be effective. For bark that already had a rather high agglomeration temperature, no effect could however be seen, although S was enriched in coating layers and agglomerate necks. In order to determine if the resulting agglomerate neck compositions agreed with the agglomeration temperatures found in the CFBA experiments, the melting behaviour for the average compositions of the agglomerate necks was evaluated using FactSage 5.4.


36

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

700 750 800 850 900 950 1000 1050 1100Temperature

Wt-

% M

elt

Bark - no additiveBark - elementary SOlive residue - no additiveOlive residue - elementary SOlive residue - elementary S - sec airOlive residue - SO2Olive residue - low SO2Olive residue - Am. Sulph.Aggl temp barkAggl olive residue no or low SAggl olive residue + S

Figure 18: Melt-% in function of the temperature for the different agglomerate neck compositions found for olive residue.

From Figure 18, a clearly different trend in melting behaviour can be seen for olive residue without additive and with low SO2 in comparison with olive residue in combination with higher levels of sulphur addition. This is in agreement with the results of the CFBA-method. For bark however, the equilibrium calculations show a rather different melting behaviour for the agglomerate necks with and without sulphur addition. However, the amount of melt is not increasing to as high levels as for the olive residue samples, which might be the explanation why agglomeration only occurred at higher temperatures. Flue gas measurements close to the bed area showed significant increases of SO2 and HCl when sulphur was added to the fuel. The measurement uncertainty being relatively large, the mass balance for Cl could not be closed completely. However, roughly calculated it seems that all Cl was converted into HCl upon addition of S. This confirms that sulphur not only reacts with alkali-chlorides in gas or aerosol phase to form sulphate and HCl, but is also sufficiently available to be enriched in the bed particle coating. Only for ammonium sulphate, the SO2 and HCl levels were low and more in the range of the untreated sample. Agglomeration temperature was however in the same range as for the other sulphur- treated samples. A possible explanation is that spraying a liquid ammonium sulphate solution on the pellets probably caused some of the ammonium sulphate to remain on the plastic underground used, thereby reducing the total amount of sulphur introduced into the reactor. Further understanding of the mechanisms can be obtained by modelling the average content of potassium, calcium and silica in the outer coatings for bark and olive residue with increasing levels of sulphur.


37

SiO2(d-SLAGB)

K2O(d-SLAGB)

SiO2(s2)

K2SO4(s2)

CaSiO3(s) CaSO4(s)

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30

SiO2(d-SLAGB)CaO(d-SLAGB)K2O(d-SLAGB)K2SO4(d-SLAGB)CaSO4(d-SLAGB)SiO2(s2)K2SO4(s2)CaSiO3(s)CaSO4(s)Level of S addition

Figure 19: The results of the thermo-chemical equilibrium calculations (mole, y-axis) of Ca-K-Si in the outer layer of bed particles upon bark combustion when increasing amounts of sulphur are added (mole, x-axis) ; calculated at 800ºC

Figure 19 shows that according to thermo-chemical equilibrium, sulphur first reacts with silica and potassium oxide slag phases to form solid SiO2 and potassium sulphate. At higher levels of sulphur, wollastonite (CaSiO3) forms solid SiO2 and CaSO4 (calcium sulphate). The approximate level of sulphur found in the outer layer of the bed particle coatings is indicated with a vertical line.

SiO2(d-SLAGB)

K2O(d-SLAGB)

K2Si4O9(s2)

K2SO4(s2)

K2Si2O5(s3)

SiO2(s4)

CaSiO3(s2) CaSO4(s)

0

10

20

30

40

50

60

0 5 10 15 20 25 30

SiO2(d-SLAGB)CaO(d-SLAGB)K2O(d-SLAGB)K2SO4(d-SLAGB)CaSO4(d-SLAGB)K2Si4O9(s2)K2SO4(s2)K2Si2O5(s3)SiO2(s4)CaSiO3(s2)CaSO4(s)Level of S addition

Figure 20: The results of the thermo-chemical equilibrium calculations (mole, y-axis) of K-Ca-Si in the outer layer of bed particles upon olive residue combustion when increasing amounts of sulphur are added (mole, x-axis) ; calculated at 760ºC


38

For olive residue, sulphur seems to react first with K2Si2O5 (s) leading to some increase of the oxide slag-phase. Upon further addition of sulphur, slag-oxides of silica and potassium disappear to form SiO2 and K2Si4O9(s) and potassium sulphate. Wollastonite forms calcium sulphate.


39

7 Conclusions

7.1 Idbäcken fuel

• The potential risk for bed agglomeration with the tested fuel mix used at Idbäcken is evaluated to be rather low. The fuel is more similar to forestry residue than to waste wood (see also Table 5). However, Zn, typically enriched in demolition wood, was not analysed on the fuel, but it did not appear in bed particle coatings or agglomerates. The bench-scale CFBA method indicated an initial agglomeration temperature of over 1020ºC. Variations in fuel compositions can however lead to lowered agglomeration temperatures but are very difficult to control.

• The agglomeration temperature of full-scale bed material samples resulted in agglomeration temperatures around 930ºC, which is still above the normal operation temperature of the furnace. Differences compared to laboratory experiments are probably caused by fuel variations.

• Addition of coal in a 90/10 biomass/coal ratio (w-%) resulted in a significant increase of the agglomeration temperature. This can be due to the higher S and Ca content in coal. These elements are found to be enriched in the coating layers and agglomerate necks

• Treatment of the return flow of bed material with SO2 has not been effective: interaction between bed material and sulphur takes probably place during fuel combustion. However the effect of other forms of sulphur remains to be assessed.

7.2 Model fuels

• HCl is formed upon addition of sulphur to the combustion area, indicating that enough sulphur was added for reaction with fuel alkali to take place and even other positive effects may be expected (such as less and higher melting deposits in the super heater area)

• For calcium-rich fuels, the effect of addition of sulphur on the agglomeration temperature was found to be limited in practice, although thermo-chemical equilibrium calculations indicated a lower content of molten phases; for potassium-rich fuels, the agglomeration temperature could be effectively increased by sufficient amounts of sulphur addition

• Melting behaviour calculated on agglomerate necks agreed fairly well with the agglomeration temperatures from the CFBA experiments

• According to the thermo-chemical equilibrium, reactions of sulphur with potassium are prioritised to reactions with calcium

• The sulphur levels tested in this work were chosen in order to ascertain a positive effect on agglomeration characteristics and are higher (up to 10 times) than the industrially acceptable levels; further experiments should be performed to find the breaking point at which level sulphur addition starts to have a positive effect on agglomeration temperature

According to the results of the SEM-analysis, sulphur is enriched in the attack and coating layers formed on the bed particles. The concentration of K and Ca is however not changed upon sulphur addition in comparison to the untreated bed particles. The increase of sulphur is significant. The thickness and morphology of the bed particle coating layers was not influenced by sulphur addition. Therefore, no evidence could be found for the hypothesis that alkali- sulphates are elutriated up an out from the bed area, thereby preventing the reaction of alkali with the bed material and reducing the risk for agglomeration. In stead, the bed material seems to bind alkali to the same extent and sulphur is in some way interacting with the bed particle coatings. Theoretically, the possible mechanisms behind this interaction can be condensed into three pathways:

1. Potassium reacts directly with sulphur to form K2SO4 during combustion, forming big particles - these condense on the bed particles

2. Reactive forms of potassium are released from the fuel during combustion, as KCl and KOH. These react with SO3 to form small K2SO4-particles that stick to the bed particles

3. KOH reacts with the bed particles to form potassium-silicates. When sufficient sulphur is added to the combustion zone, SO3 reacts with the potassium-silicates to form K2SO4.

During SEM/EDS analysis however, no concentrated K/S areas indicating the presence of large K2SO4 particles could be discerned in the bed particle coatings and agglomerate necks. Therefore, pathway (1) is probably excluded in this case. Pathway (2) is more likely to occur although very little KCl was available in the fuel and this pathway should lead to elutriation of alkali- sulphates out of the bed area rather than interaction with the bed material. From the results of this study it is therefore concluded that pathway (3), interaction between sulphur and alkali-silicates in the bed particle coating, is the most likely to take place.


40

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Appendix Phase diagrams K2O – Al2O3 – SiO2


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CaO – K2O – SiO2


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SEM-pictures Combustion of bark

Bark – no additive – Bed particle Bark – no additive - Agglomerate

Bark – Elementary S – Bed particle Bark – Elementary S – Neck between two particles Combustion of olive residue

Olive residue – no additive – Bed particle Olive residue – no additive - Agglomerate


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Olive residue – Elementary S – Bed particle Olive residue – Elementary S - Agglomerate

Olive residue – Elementary S + sec. air – Bed particle Olive residue – Elementary S + sec. air - Agglomerate

Olive residue – SO2 – Bed particle Olive residue – SO2 - Agglomerate


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Combustion of Idbäcken fuel

Idbäcken fuel – no additive – Bed particle Idbäcken fuel – no additive - Agglomerate

Idbäcken fuel – coal – Bed particle Idbäcken fuel – coal – Agglomerate

the role of sulphur in preventing bed agglomeration during ... · increase sharply with...

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